Display Device and Driving Method Thereof

ABSTRACT

Power consumption of a display device is reduced. Display quality of a display device is improved. A high-quality image can be displayed regardless of a usage environment. The display device includes a first display element, a second display element, and a control portion. The first display device reflects visible light. The second display element emits visible light. 
     The control portion is configured to drive the first display element and the second display element at the same time such that a maximum value of luminance of light emitted from the second display element is greater than or equal to 1% and less than or equal to 50% of maximum luminance on the assumption that maximum luminance of light which is emitted from the second display element is 100%.

This application is a continuation of copending U.S. application Ser.No. 15/623,736, filed on Jun. 15, 2017 which is incorporated herein byreference.

TECHNICAL FIELD

One embodiment of the present invention relates to a display device. Oneembodiment of the present invention relates to a method for driving thedisplay device.

Note that one embodiment of the present invention is not limited to theabove technical field. Examples of the technical field of one embodimentof the present invention disclosed in this specification and the likeinclude a semiconductor device, a display device, a light-emittingdevice, a power storage device, a memory device, an electronic device, alighting device, an input device, an input/output device, a drivingmethod thereof, and a manufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor, a semiconductor circuit, an arithmeticdevice, a memory device, and the like are each an embodiment of thesemiconductor device. In addition, an imaging device, an electro-opticaldevice, a power generation device (e.g., a thin film solar cell and anorganic thin film solar cell), and an electronic device each may includea semiconductor device.

BACKGROUND ART

As one of display devices, there is a liquid crystal display deviceprovided with a liquid crystal element. For example, an active matrixliquid crystal display device, in which pixel electrodes are arranged ina matrix and transistors are used as switching elements connected torespective pixel electrodes, has attracted attention.

For example, an active matrix liquid crystal display device includingtransistors, in which a metal oxide is used for a channel formationregion, as switching elements connected to respective pixel electrodesis already known (Patent Documents 1 and 2).

It is known that an active matrix liquid crystal display device isclassified into two major types: transmissive type and reflective type.

In a transmissive liquid crystal display device, a backlight such as acold cathode fluorescent lamp or a light-emitting diode (LED) is used,and optical modulation action of liquid crystal is utilized to selectone of the two states: a state where light from the backlight passesthrough liquid crystal to be output to the outside of the liquid crystaldisplay device and a state where light is not output to the outside ofthe liquid crystal display device, whereby a bright or dark image isdisplayed. Furthermore, those images are combined to display an image.

In a reflective liquid crystal display device, a state in which externallight, in other words, incident light is reflected at a pixel electrodeand output to the outside of the device or a state in which incidentlight is not output to the outside of the device is selected usingoptical modulation action of a liquid crystal, whereby bright and darkimages are displayed. Furthermore, those displays are combined todisplay an image. Compared with the transmissive liquid crystal displaydevice, the reflective liquid crystal display device has the advantageof low power consumption since the backlight is not used.

REFERENCE Patent Documents

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

DISCLOSURE OF INVENTION

Electronic devices including display devices are required to reducetheir power consumption. In particular, since the energy consumption ofdisplay devices accounts for a significant proportion in devices indevices using batteries as power sources, such as mobile phones,smartphones, tablet terminals, smart watches, and notebook personalcomputers, low power consumption of display devices is required.

An object of one embodiment of the present invention is to reduce powerconsumption of a display device. Another object of one embodiment of thepresent invention is to improve display quality of a display device.Another object of one embodiment of the present invention is to displaya high-quality image regardless of a usage environment.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects can be derived fromthe description of the specification and the like.

One embodiment of the present invention is a method for driving adisplay device. The display device includes a first display element thatreflects visible light and a second display element that emits visiblelight. When the first display element and the second display element aredriven at the same time to display an image, a maximum value ofluminance of light emitted from the second display element is greaterthan or equal to 5% and less than or equal to 50% of maximum luminanceon the assumption that the maximum luminance of light which is emittedfrom the second display element is 100%.

Another embodiment of the present invention is a method for driving adisplay device. The display device includes a first display element thatreflects visible light and a second display element that emits visiblelight. When the first display element is not driven and the seconddisplay element is driven to display an image, a maximum value ofluminance of light emitted from the second display element is greaterthan or equal to 50% and less than or equal to 100% of maximum luminanceon the assumption that the maximum luminance of light which is emittedfrom the second display element is 100%.

Another embodiment of the present invention is a display deviceincluding a first display element, a second display element, and acontrol portion. The first display element reflects visible light. Thesecond display element emits visible light. The control portion isconfigured to drive the first display element and the second displayelement at the same time such that a maximum value of luminance of lightemitted from the second display element is greater than or equal to 5%and less than or equal to 50% of maximum luminance on the assumptionthat the maximum luminance of light which is emitted from the seconddisplay element is 100%.

Another embodiment of the present invention is a display deviceincluding a first display element, a second display element, and acontrol portion. The first display element reflects visible light. Thesecond display element emits visible light. The control portion isconfigured to not drive the first display element and to drive thesecond display element such that a maximum value of luminance of lightemitted from the second display element is greater than or equal to 50%and less than or equal to 100% of maximum luminance on the assumptionthat the maximum luminance of light which is emitted from the seconddisplay element is 100%.

In the above, the second display element preferably includes anisland-shaped first light-emitting layer that emits light of apredetermined color. At that time, a plurality of second displayelements is arranged at a resolution of higher than or equal to 50 ppiand lower than 300 ppi.

In the above, it is preferable that the display device further include acoloring layer that is provided to overlap with the second displayelement. The second display element preferably includes a secondlight-emitting layer that emits white light. At that time, a pluralityof second display elements is preferably arranged at a resolution ofhigher than or equal to 300 ppi and lower than or equal to 3000 ppi,preferably higher than or equal to 500 ppi and lower than or equal to2500 ppi.

According to one embodiment of the present invention, power consumptionof a display device can be reduced. Alternatively, display quality of adisplay device can be improved. Alternatively, a high-quality image canbe displayed regardless of a usage environment.

Note that one embodiment of the present invention does not necessarilyachieve all the effects listed above. Other effects will be apparentfrom and can be derived from the description of the specification, thedrawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a display device of one embodiment.

FIGS. 2A to 2C illustrate pixel units of one embodiment.

FIGS. 3A, 3B1 and 3B2 illustrate a display panel of one embodiment.

FIG. 4 is a circuit diagram of a display panel of one embodiment.

FIG. 5A is a circuit diagram of a display panel of one embodiment, andFIG. 5B is a structure example of a pixel of one embodiment.

FIG. 6 is a structure example of a display panel of one embodiment.

FIG. 7 is a structure example of a display panel of one embodiment.

FIG. 8 is a structure example of a display panel of one embodiment.

FIGS. 9A1, 9A2, 9B1, 9B2, 9C1, and 9C2 are structure examples oftransistors of one embodiment.

FIGS. 10A1, 10A2, 10A3, 10B1, and 10B2 are structure examples oftransistors of one embodiment.

FIGS. 11A1, 11A2, 11A3, 11B1, 11B2, 11C1, and 11C2 are structureexamples of transistors of one embodiment.

FIGS. 12A to 12F illustrate examples of electronic devices and alighting device of one embodiment. FIGS. 13A to 13I illustrate examplesof electronic devices of one embodiment.

FIGS. 14A to 14F illustrate examples of electronic devices of oneembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments are described in detail with reference to the drawings. Notethat the present invention is not limited to the description below, andit is easily understood by those skilled in the art that various changesand modifications can be made without departing from the spirit andscope of the present invention. Thus, the present invention should notbe construed as being limited to the description in the followingembodiments.

Note that in structures of the present invention described below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and a description thereofis not repeated. Further, the same hatching pattern is used for portionshaving similar functions, and the portions are not especially denoted byreference numerals in some cases.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such a scale.

Note that in this specification and the like, ordinal numbers such as“first,” “second,” and the like are used in order to avoid confusionamong components and do not limit the number.

A transistor is a kind of semiconductor elements and can achieveamplification of current and voltage, switching operation forcontrolling conduction and non-conduction, and the like. A transistor inthis specification includes an insulated-gate field effect transistor(IGFET) and a thin film transistor (TFT).

Embodiment 1

In this embodiment, a display device of one embodiment of the presentinvention and a driving method thereof are described.

In the display device of one embodiment of the present invention, afirst display element that reflects visible light and a second displayelement that emits visible light are mixed.

The display device has a function of displaying an image utilizing oneor both of first light reflected by the first display element and secondlight emitted from the second display element. Alternatively, thedisplay device has a function of expressing gray scales by individuallycontrolling the amount of first light reflected by the first displayelement and the amount of second light emitted from the second displayelement.

It is preferable that the display device have a structure including afirst pixel expressing gray scales by controlling the amount of lightreflected from the first display element and a second pixel expressinggray scales by controlling the amount of light emitted from the seconddisplay element. For example, the first pixels are arranged in a matrixand the second pixels are arranged in a matrix to form a displayportion.

The number of the first pixels is preferably the same as that of thesecond pixels, and the first pixels and the second pixels are preferablyarranged in a display region with the same pitch. Here, the first pixeland the second pixel adjacent to each other can be collectively referredto as a pixel unit.

Furthermore, the first pixels and the second pixels are preferably mixedin the display region of the display device. Accordingly, as describedlater, an image displayed by a plurality of first pixels, an imagedisplayed by a plurality of second pixels, and an image displayed byboth the plurality of first pixels and the plurality of second pixelscan be displayed in the same display region.

As the first display element included in the first pixel, an elementthat performs display by reflecting external light can be used. Such anelement does not include a light source and thus power consumption indisplay can be significantly reduced.

As the first display element, a reflective liquid crystal element can betypically used. As the first display element, other than a Micro ElectroMechanical Systems (MEMS) shutter element and an optical interferencetype MEMS element, an element using a microcapsule method, anelectrophoretic method, an electrowetting method, an Electronic LiquidPowder (registered trademark) method, or the like can be used.

As the second display element included in the second pixel, an elementincluding a light source and performing display using light from thelight source can be used. Specifically, it is preferable to use anelectroluminescence element in which light can be extracted from alight-emitting substance by application of an electric field. Since theluminance and the chromaticity of light emitted from such a pixel arenot affected by external light, an image with high color reproducibility(a wide color gamut) and a high contrast, i.e., a vivid image can bedisplayed.

As the second display element, a self-luminous light-emitting elementsuch as an organic light-emitting diode (OLED), a light-emitting diode(LED), and a quantum-dot light-emitting diode (QLED) can be used.Alternatively, a combination of a backlight that is a light source and atransmissive liquid crystal element controlling the amount of lighttransmitted from the backlight may be used as the display elementincluded in the second pixel.

The first pixel can include, for example, subpixels emitting light ofwhite (W), or subpixels emitting light of three colors of red (R), green(G), and blue (B), respectively. Similarly, the second pixel caninclude, for example, subpixels emitting light of white (W), orsubpixels emitting light of three colors of red (R), green (G), and blue(B), respectively. Note that the first pixel and the second pixel mayeach include subpixels of four colors or more. As the number ofsubpixels is increased, power consumption can be reduced and colorreproducibility can be improved.

In one embodiment of the present invention, a first mode in which animage is displayed by the first pixels, a second mode in which an imageis displayed by the second pixels, and a third mode in which an image isdisplayed by the first pixels and the second pixels can be switched.

The first mode is a mode in which an image is displayed utilizing lightreflected from the first display element. In the first mode, a lightsource is not necessary and thus the first mode is a driving mode withextremely low power consumption. The first mode is effective in the casewhere, for example, external light has a sufficiently high illuminanceand is white light or light near white light. The first mode is adisplay mode appropriate for displaying text data, such as that of abook or that of a document. Use of reflected light enables eye-friendlydisplay, thereby mitigating eye fatigue.

The second mode is a mode in which an image is displayed utilizing lightemitted from the second display element. Thus, an extremely vivid image(high contrast and high color reproducibility) can be displayedregardless of the illuminance and chromaticity of external light. Forexample, the second mode is effective in the case where the illuminanceof external light is extremely low, such as during the night or in adark room. When a bright image is displayed under weak external light, auser may feel that the image is too bright. To prevent this, an imagewith reduced luminance is preferably displayed in the second mode. Thus,not only a reduction in the luminance but also low power consumption canbe achieved. The second mode is a mode suitable for obtaining a vividimage and a smooth moving image.

The third mode is a mode in which display is performed using both lightreflected from the first display element and light emitted from thesecond display element. Specifically, the display device is driven sothat light emitted from the first pixel and light emitted from thesecond pixel adjacent to the first pixel are mixed to express one color.Accordingly, a more vivid image than that in the first mode can bedisplayed and power consumption can be made lower than that in thesecond mode. For example, the third mode is effective when theilluminance of external light is relatively low such as under indoorillumination or in the morning or evening, or when the external lightdoes not represent a white chromaticity. Furthermore, the use of lightobtained by mixing reflected light and emitted light makes it possibleto display an image that gives a viewer the impression of seeing apicture.

Here, in the third mode in which an image is displayed using both lightreflected from the first display element and light emitted from thesecond display element, luminance of the second display element ispreferably reduced. For example, on the assumption that a maximum valueof luminance (also referred to as maximum luminance) of light which isemitted from the second display element is 100%, a maximum value ofluminance of light emitted from the second display element in the thirdmode is preferably greater than or equal to 5% and less than or equal to50%, preferably greater than or equal to 1% and less than or equal to60% of the maximum luminance. Accordingly, display with low powerconsumption can be achieved, a picture-like image can be displayed, andeye-friendly display can be performed.

Furthermore, in the second mode in which only a display element thatemits visible light is used, luminance of the display element that emitsvisible light is preferably increased. For example, a maximum value ofluminance of light emitted from the second display element in the secondmode can be 100%, or greater than or equal to 50% and less than or equalto 100%, preferably greater than or equal to 60% and less than or equalto 100% of the maximum luminance. Accordingly, a vivid image can bedisplayed even in a place with bright external light.

Here, the maximum value of luminance of light emitted from the seconddisplay element can be expressed by a dynamic range of the seconddisplay element. That is, the dynamic range of the second displayelement in the third mode can be set narrower than that in the secondmode. For example, the dynamic range of the second display element inthe third mode can be set at greater than or equal to 5% and less thanor equal to 50%, preferably greater than or equal to 1% and less than orequal to 60% of the dynamic range of the second display element in thesecond mode.

More specifically, the display device can include a display panelincluding the first and second pixels, and a control portion. Thecontrol portion generates and outputs a first gray level and a secondgray level to the first pixel and the second pixel, respectively, on thebasis of image data input from the outside. Here, the image data is dataincluding a gray level corresponding to each pixel unit, and an imagesignal such as a video signal is given as an example.

Note that the control portion may have a function of selecting theabove-described display modes on the basis of external light illuminanceand the like.

A more specific example of one embodiment of the present invention isdescribed below with reference to drawings.

Structure Example of Display Device

FIG. 1 is a block diagram of a display device 10 of one embodiment ofthe present invention. The display device 10 includes a control portion11, a driver portion 13, and a display portion 14. The display device 10may have a photometric portion which measures external light illuminanceand the like.

The control portion 11 includes an arithmetic portion 31.

The display portion 14 includes a plurality of pixel units 20 arrangedin a matrix. The pixel unit 20 includes a first pixel 21 and a secondpixel 22.

FIG. 1 shows an example where the first pixel 21 and the second pixel 22each include display elements corresponding to three colors of red (R),green (G), and blue (B).

The first pixel 21 includes a display element 21R corresponding to red(R), a display element 21G corresponding to green (G), and a displayelement 21B corresponding to blue (B). The display elements 21R, 21G,and 21B each utilize reflection of external light.

The second pixel 22 includes a display element 22R corresponding to red(R), a display element 22G corresponding to green (G), and a displayelement 22B corresponding to blue (B). The display elements 22R, 22G,and 22B each utilize light of a light source.

The driver portion 13 includes a circuit for driving the plurality ofpixel units 20 in the display portion 14. Specifically, the driverportion 13 includes a circuit that supplies a signal including a graylevel, a scan signal, a power supply potential, and the like to thefirst pixel 21 and the second pixel 22 included in the pixel unit 20.The driver portion 13 includes a signal line driver circuit and a scanline driver circuit, for example.

A image signal S0 including image data is input to the control portion11 from the outside. The control portion 11 generates two signals (asignal S1 and a signal S2) including gray levels supplied to each pixelunit 20 in the display portion 14, and outputs the signals to the driverportion 13. The control portion 11 generates a timing signal such as aclock signal or a start pulse signal in addition to the signals Si andS2 and outputs the signals to the driver portion 13.

The signal S1 includes a gray level supplied to the first pixel 21 inthe pixel unit 20. Here, the signal S1 includes data of three graylevels supplied to the respective display elements 21R, 21G, and 21B inone pixel unit 20.

The signal S2 includes gray levels supplied to the second pixel 22 inthe pixel unit 20. Here, the signal S2 includes data of three graylevels supplied to the respective display elements 22R, 22G, and 22B inone pixel unit 20.

The signals S1 and S2 each may be a serial signal transmitted throughone signal line or a parallel signal transmitted through a plurality ofsignal lines.

The control portion 11 is configured to select one of the first mode,the second mode, and the third mode described below, generate thesignals S1 and S2 based on the respective modes, and output the signalsto the driver portion 13.

For example, in the third mode in which both a display element thatreflects external light and a display element that emits light aredriven to display an image, the control portion 11 is configured todrive the display element that reflects external light and the displayelement that emits light at the same time such that a maximum value ofluminance of light emitted from the display element that emits light isgreater than or equal to 5% and less than or equal to 50%, preferablygreater than or equal to 1% and less than or equal to 60% of maximumluminance on the assumption that the maximum luminance of light which isemitted from the display element that emits light is 100%.

Furthermore, for example, in the second mode in which the displayelement that emits light is driven to display an image, the controlportion 11 is configured to not drive the display element that reflectsexternal light and to drive the display element that emits light suchthat the maximum value of luminance of light emitted from the displayelement that emits light is 100%, or greater than or equal to 50% andless than or equal to 100%, preferably greater than or equal to 60% andless than or equal to 100% of maximum luminance on the assumption thatthe maximum luminance of light which is emitted from the display elementthat emits light is 100%.

Here, a microprocessor such as a graphics processing unit (GPU) can beused as the arithmetic portion 31, for example. Furthermore, such amicroprocessor may be obtained with a programmable logic device (PLD)such as a field programmable gate array (FPGA) or a field programmableanalog array (FPAA).

Here, the image signal S0 may be generated by a central processing unit(CPU) or the like provided separately from the display device 10 andsupplied to the control portion 11. Alternatively, the arithmeticportion 31 may serve as a CPU and have a function of generating theimage signal S0.

The image signal S0 input from the outside may be a signal that hasalready been subjected to gamma correction. The arithmetic portion 31may have a function of performing the correction. The arithmetic portion31 may generate the signals S1 and S2 based on a signal resulting fromcorrection being performed on the image signal S0 or may correct each ofgenerated signals S1 and S2.

The arithmetic portion 31 interprets and executes instructions fromprograms to process various kinds of data and control programs. Theprograms executed by the processor may be stored in a memory regionincluded in the processor or a memory device which is additionallyprovided.

The arithmetic portion 31 may include a main memory. The main memory caninclude a volatile memory, such as a random access memory (RAM), and anonvolatile memory, such as a read only memory (ROM).

For example, a dynamic random access memory (DRAM) is used for the RAM.A memory space as a workspace for the arithmetic portion 31 is virtuallyallocated for the RAM and used in the arithmetic portion 31. Anoperating system, an application program, a program module, programdata, and the like stored in a memory device provided outside are loadedinto the RAM and executed. The data, program, and program module whichare loaded into the RAM are directly accessed and operated by thearithmetic portion 31.

The control portion 11 may be mounted on a circuit board such as aprinted circuit, and the driver portion 13 may be provided over asubstrate over which the display portion 14 is formed. Here, the circuitboard and the driver portion 13 are connected to each other via aflexible printed circuit (FPC) or the like. Furthermore, the driverportion 13 may be formed over a substrate over which the display portion14 is formed through the same step as transistors and the like includedin the display portion 14, and part or all of the driver portion 13 maybe mounted on the substrate as an integrated circuit (IC).Alternatively, the control portion 11 and the driver portion 13 may bemounted on the substrate as one or more ICs. Alternatively, the controlportion 11 and the driver portion 13 may be formed over a substrate overwhich the display portion 14 is formed through the same step astransistors included in the display portion 14.

That is the description of the structure examples of the display device.

Structure Example of Pixel Unit

Next, the pixel unit 20 is explained with reference to FIGS. 2A to 2C.FIGS. 2A to 2C are schematic views illustrating structure examples ofthe pixel unit 20.

The first pixel 21 includes the display elements 21R, 21G, and 21B. Thedisplay element 21R reflects external light and emits red light R1, tothe display surface side, with luminance in accordance with a gray levelcorresponding to red included in the first gray level input to the firstpixel 21. Similarly, the display element 21G and the display element 21Bemit green light G1 and blue light B1, respectively, to the displaysurface side.

The second pixel 22 includes the display elements 22R, 22G, and 22B. Thedisplay element 22R includes a light source and emits red light R2, tothe display surface side, with luminance in accordance with a gray levelcorresponding to red included in the second gray level input to thesecond pixel 22. Similarly, the display element 22G and the displayelement 22B emit green light G2 and blue light B2, respectively, to thedisplay surface side.

Third Mode

FIG. 2A illustrates an example of an operation mode in which both thedisplay elements 21R, 21G, and 21B that reflect external light and thedisplay elements 22R, 22G, and 22B that emit light are driven to displayan image. As illustrated in FIG. 2A, the pixel unit 20 can emit light 25of a predetermined color to the display surface side by mixing light ofsix colors, the light R1, the light G1, the light B1, the light R2, thelight G2, and the light B2.

At this time, luminance of each of the display elements 22R, 22G, and22B is preferably lowered. For example, on the assumption that themaximum value of luminance (also referred to as maximum luminance) oflight which is emitted from each of the display elements 22R, 22G, and22B is 100%, the maximum value of luminance of light emitted from eachof the display elements 22R, 22G, and 22B in the third mode is greaterthan or equal to 5% and less than or equal to 50%, preferably greaterthan or equal to 1% and less than or equal to 60% of the maximumluminance. Accordingly, display with low power consumption can beachieved, a picture-like image can be displayed, and eye-friendlydisplay can be achieved.

First Mode

FIG. 2B illustrates an example of an operation mode in which the displayelements 21R, 21G, and 21B that reflect external light are driven todisplay an image. As illustrated in FIG. 2B, in the case whereilluminance of external light is sufficiently high, for example, thepixel unit 20 can emit the light 25 of a predetermined color to thedisplay surface side by not driving the second pixel 22 and mixing onlylight (the light R1, the light G1, and the light B1) from the firstpixel 21. Thus, driving with extremely low power consumption can beperformed.

Second Mode

FIG. 2C illustrates an example of an operation mode in which the displayelements 22R, 22G, and 22B are driven to display an image. Asillustrated in FIG. 2C, in the case where illuminance of external lightis extremely low, for example, the pixel unit 20 can emit the light 25of a predetermined color to the display surface side by not driving thefirst pixel 21 and mixing only light (the light R2, the light G2, andthe light B2) from the second pixel 22. Accordingly, a clear image canbe displayed. Furthermore, luminance is lowered when illuminance ofexternal light is low, which can prevent a user from feeling glare andreduce power consumption.

At this time, the luminance of the display elements that emit visiblelight in the second mode is preferably higher than that in the thirdmode. For example, the maximum value of luminance of light emitted fromeach of the display elements 22R, 22G, and 22B in the second mode can be100%, or greater than or equal to 50% or less than or equal to 100%,preferably greater than or equal to 60% or less than or equal to 100% ofthe maximum luminance. Accordingly, a vivid image can be displayed evenin a place with bright external light.

Here, the maximum value of luminance of light emitted from each of thedisplay elements 22R, 22G, and 22B can be replaced with a dynamic rangethereof. That is, the dynamic range of each of the display elements 22R,22G, and 22B in the third mode can be set narrower than that in thesecond mode. For example, the dynamic range of each of the displayelements 22R, 22G, and 22B in the third mode is set to be greater thanor equal to 5% and less than or equal to 50%, preferably greater than orequal to 1% and less than or equal to 60% of the dynamic range of thesecond mode.

The above is the description of the structure example of the pixel unit20.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 2

An example of a display panel that can be used for the display portionor the like of the display device of one embodiment of the presentinvention is described below. The display panel described below as anexample includes both a reflective liquid crystal element and alight-emitting element and can display an image both in a transmissivemode and in a reflective mode.

Structure Example

FIG. 3A is a block diagram illustrating an example of the structure of adisplay device 400. The display device 400 includes a plurality ofpixels 410 that are arranged in a matrix in a display portion 362. Thedisplay device 400 also includes a circuit GD and a circuit SD. Inaddition, the display device 400 includes a plurality of wirings G1, aplurality of wirings G2, a plurality of wirings ANO, and a plurality ofwirings CSCOM, which are electrically connected to the circuit GD andthe plurality of pixels 410 arranged in a direction R. Moreover, thedisplay device 400 includes the plurality of pixels 410 arranged in adirection C, and a plurality of wirings S1 and a plurality of wirings S2that are electrically connected to the circuit SD.

Although the display device includes one circuit GD and one circuit SDhere for simplification, the circuit GD and the circuit SD for driving aliquid crystal element and the circuit GD and the circuit SD for drivinga light-emitting element may be separately provided.

The pixel 410 includes a reflective liquid crystal element and alight-emitting element. In the pixel 410, the liquid crystal element andthe light emitting element partly overlap with each other.

FIG. 3B1 illustrates a structure example of an electrode 311 b includedin the pixel 410. The electrode 311 b serves as a reflective electrodeof the liquid crystal element in the pixel 410. The electrode 311 b hasan opening 451.

In FIG. 3B1, the light-emitting element 360 in a region overlapping withthe electrode 311 b is denoted by a dashed line. The light-emittingelement 360 overlaps with the opening 451 included in the electrode 311b. Thus, light from the light-emitting element 360 is emitted to adisplay surface side through the opening 451.

In FIG. 3B1, the pixels 410 adjacent in the direction R correspond todifferent colors. As illustrated in FIG. 3B1, the openings 451 arepreferably provided in different positions in the electrodes 311 b so asnot to be aligned in the two pixels adjacent to each other in thedirection R. This allows the two light-emitting elements 360 to be apartfrom each other, thereby preventing light emitted from thelight-emitting element 360 from entering a coloring layer in theadjacent pixel 410 (such a phenomenon is also referred to as crosstalk).Furthermore, since the two adjacent light-emitting elements 360 can bearranged apart from each other, a high-resolution display device isachieved even when EL layers of the light-emitting elements 360 areseparately formed with a shadow mask or the like.

Alternatively, arrangement illustrated in FIG. 3B2 may be employed.

If the ratio of the total area of the opening 451 to the total areaexcept for the opening is too large, display performed using the liquidcrystal element is dark. If the ratio of the total area of the opening451 to the total area except for the opening is too small, displayperformed using the light-emitting element 360 is dark.

If the area of the opening 451 in the electrode 311 b serving as areflective electrode is too small, light emitted from the light-emittingelement 360 is not efficiently extracted.

The opening 451 may have a polygonal shape, a quadrangular shape, anelliptical shape, a circular shape, a cross-like shape, a stripe shape,a slit-like shape, or a checkered pattern, for example. The opening 451may be provided close to the adjacent pixel. Preferably, the opening 451is provided close to another pixel emitting light of the same color, inwhich case crosstalk can be suppressed.

Circuit Structure Example

FIG. 4 is a circuit diagram illustrating a structure example of thepixel 410. FIG. 4 shows two adjacent pixels 410.

The pixel 410 includes a switch SW1, a capacitor C1, a liquid crystalelement 340, a switch SW2, a transistor M, a capacitor C2, thelight-emitting element 360, and the like. The pixel 410 is electricallyconnected to the wiring G1, the wiring G2, the wiring ANO, the wiringCSCOM, the wiring S1, and the wiring S2. FIG. 4 illustrates a wiringVCOM1 electrically connected to the liquid crystal element 340 and awiring VCOM2 electrically connected to the light-emitting element 360.

FIG. 4 illustrates an example in which a transistor is used as each ofthe switches SW1 and SW2.

A gate of the switch SW1 is connected to the wiring G1. One of a sourceand a drain of the switch SW1 is connected to the wiring S1, and theother of the source and the drain is connected to one electrode of thecapacitor C1 and one electrode of the liquid crystal element 340. Theother electrode of the capacitor C1 is connected to the wiring CSCOM.The other electrode of the liquid crystal element 340 is connected tothe wiring VCOM1.

A gate of the switch SW2 is connected to the wiring G2. One of a sourceand a drain of the switch SW2 is connected to the wiring S2, and theother of the source and the drain is connected to one electrode of thecapacitor C2 and a gate of the transistor M. The other electrode of thecapacitor C2 is connected to one of a source and a drain of thetransistor M and the wiring ANO. The other of the source and the drainof the transistor M is connected to one electrode of the light-emittingelement 360. The other electrode of the light-emitting element 360 isconnected to the wiring VCOM2.

FIG. 4 illustrates an example in which the transistor M includes twogates between which a semiconductor is provided and which are connectedto each other. This structure can increase the amount of current flowingthrough the transistor M.

The wiring G1 can be supplied with a signal for changing the on/offstate of the switch SW1. A predetermined potential can be supplied tothe wiring VCOM1. The wiring S1 can be supplied with a signal forchanging the orientation of liquid crystals of the liquid crystalelement 340. A predetermined potential can be supplied to the wiringCSCOM.

The wiring G2 can be supplied with a signal for changing the on/offstate of the switch SW2. The wiring VCOM2 and the wiring ANO can besupplied with potentials having a difference large enough to make thelight-emitting element 360 emit light. The wiring S2 can be suppliedwith a signal for changing the conduction state of the transistor M.

In the case of performing display in the reflective mode, the pixel 410shown in FIG. 4 can display an image by being driven with signalssupplied to the wiring G1 and the wiring S1 and by utilizing opticalmodulation of the liquid crystal element 340. Furthermore, in the caseof performing display in the transmissive mode, the pixel 410 shown inFIG. 4 can display an image by being driven with signals supplied to thewiring G2 and the wiring S2 and by making the light-emitting element 360emit light. Furthermore, an image can be displayed while driving thepixel in both modes with the signals supplied to the wiring G1, thewiring G2, the wiring S1, and the wiring S2.

Although FIG. 4 illustrates an example in which one liquid crystalelement 340 and one light-emitting element 360 are provided in one pixel410, one embodiment of the present invention is not limited thereto.FIG. 5A illustrates an example in which one liquid crystal element 340and four light-emitting elements 360 (light-emitting elements 360 r, 360g, 360 b, and 360 w) are provided in one pixel 410. The pixel 410 inFIG. 5A is capable of full color display by one pixel, which isdifferent from the pixel in FIG. 4.

In FIG. 5A, in addition to the example of FIG. 4, a wiring G3 and awiring S3 are connected to the pixel 410.

In the example of FIG. 5A, light-emitting elements emitting red light(R), green light (G), blue light (B), and white light (W) can be usedfor the four light-emitting elements 360, for example. Furthermore, asthe liquid crystal element 340, a reflective liquid crystal elementemitting white light can be used. In the case of performing display inthe reflective mode, white display with high reflectivity can beperformed. In the case of performing display in the transmissive mode,images can be displayed with a higher color rendering property at lowpower consumption.

FIG. 5B illustrates a structure example of the pixel 410. The pixel 410includes the light-emitting element 360 w which overlaps with theopening of an electrode 311 and the light-emitting element 360 r, 360 g,and 360 b which are provided in the periphery of the electrode 311. Itis preferable that the light-emitting elements 360 r, 360 g, and 360 bhave almost the same light-emitting area.

Structure Example of Display Panel

FIG. 6 is a schematic perspective view illustrating a display panel 100of one embodiment of the present invention. In the display panel 100, asubstrate 51 and a substrate 61 are attached to each other. In FIG. 6,the substrate 61 is denoted by a dashed line.

The display panel 100 includes a display portion 62, a circuit 64, awiring 65, and the like. The substrate 51 is provided with the circuit64, the wiring 65, a conductive layer 111 b which serves as a pixelelectrode, and the like. In FIG. 6, an IC 73 and an FPC 72 are mountedon the substrate 51. Thus, the structure illustrated in FIG. 6 can bereferred to as a display module including the display panel 100, the FPC72, and the IC 73.

As the circuit 64, for example, a circuit functioning as a scan linedriver circuit can be used.

The wiring 65 has a function of supplying a signal or electric power tothe display portion 62 or the circuit 64. The signal or electric poweris input to the wiring 65 from the outside through the FPC 72 or fromthe IC 73.

FIG. 6 shows an example in which the IC 73 is provided on the substrate51 by a chip on glass (COG) method or the like. As the IC 73, an ICfunctioning as a scan line driver circuit, a signal line driver circuit,or the like can be used. Note that it is possible that the IC 73 is notprovided when, for example, the display panel 100 includes circuitsserving as a scan line driver circuit and a signal line driver circuitand when the circuits serving as a scan line driver circuit and a signalline driver circuit are provided outside and a signal for driving thedisplay panel 100 is input through the FPC 72. Alternatively, the IC 73may be mounted on the FPC 72 by a chip on film (COF) method or the like.

FIG. 6 also shows an enlarged view of part of the display portion 62.The conductive layers 111 b included in a plurality of display elementsare arranged in a matrix in the display portion 62. The conductive layer111 b has a function of reflecting visible light and serves as areflective electrode of the liquid crystal element 40 described later.

As illustrated in FIG. 6, the conductive layer 111 b has an opening. Thelight-emitting element 60 is provided on the substrate 51 side of theconductive layer 111 b. Light is emitted from the light-emitting element60 to the substrate 61 side through the opening in the conductive layer111 b.

Cross-Sectional Structure Example

FIG. 7 shows an example of cross sections of part of a region includingthe FPC 72, part of a region including the circuit 64, and part of aregion including the display portion 62 of the display panel illustratedin FIG. 6.

The display panel includes an insulating layer 220 between thesubstrates 51 and 61. The display panel also includes the light-emittingelement 60, a transistor 201, a transistor 205, a transistor 206, acoloring layer 134, and the like between the substrate 51 and theinsulating layer 220. Furthermore, the display panel includes the liquidcrystal element 40, the coloring layer 131 and the like between theinsulating layer 220 and the substrate 61. The substrate 61 and theinsulating layer 220 are bonded with an adhesive layer 141. Thesubstrate 51 and the insulating layer 220 are bonded with an adhesivelayer 142.

The transistor 206 is electrically connected to the liquid crystalelement 40 and the transistor 205 is electrically connected to thelight-emitting element 60. Since the transistors 205 and 206 are formedon a surface of the insulating layer 220 which is on the substrate 51side, the transistors 205 and 206 can be formed through the sameprocess.

The coloring layer 131, a light-blocking layer 132, an insulating layer121, and a conductive layer 113 serving as a common electrode of theliquid crystal element 40, an alignment film 133 b, an insulating layer117, and the like are provided over the substrate 61. The insulatinglayer 117 serves as a spacer for holding a cell gap of the liquidcrystal element 40.

Insulating layers such as an insulating layer 211, an insulating layer212, an insulating layer 213, an insulating layer 214, an insulatinglayer 215, and the like are provided on the substrate 51 side of theinsulating layer 220. Part of the insulating layer 211 functions as agate insulating layer of each transistor. The insulating layer 212, theinsulating layer 213, and the insulating layer 214 are provided to covereach transistor and the like. The insulating layer 215 is provided tocover the insulating layer 214. The insulating layers 214 and 215 eachfunction as a planarization layer. Note that an example where the threeinsulating layers, the insulating layers 212, 213, and 214, are providedto cover the transistors and the like is described here; however, oneembodiment of the present invention is not limited to this example, andfour or more insulating layers, a single insulating layer, or twoinsulating layers may be provided. The insulating layer 214 functioningas a planarization layer is not necessarily provided when not needed.

The transistors 201, 205, and 206 each include a conductive layer 221part of which functions as a gate, conductive layers 222 part of whichfunctions as a source and a drain, and a semiconductor layer 231. Here,a plurality of layers obtained by processing the same conductive filmare shown with the same hatching pattern.

The liquid crystal element 40 is a reflective liquid crystal element.The liquid crystal element 40 has a structure in which a conductivelayer 111 a, a liquid crystal 112, and the conductive layer 113 arestacked. A conductive layer 111 b which reflects visible light isprovided in contact with the surface of the conductive layer 111 a thatis on the substrate 51 side. The conductive layer 111 b includes anopening 251. The conductive layers 111 a and 113 contain a materialtransmitting visible light. In addition, an alignment film 133 a isprovided between the liquid crystal 112 and the conductive layer 111 aand the alignment film 133 b is provided between the liquid crystal 112and the conductive layer 113. A polarizing plate 130 is provided on anouter surface of the substrate 61.

In the liquid crystal element 40, the conductive layer 111 b has afunction of reflecting visible light, and the conductive layer 113 has afunction of transmitting visible light. Light that enters the substrate61 side is polarized by the polarizing plate 130, passes through theconductive layer 113 and the liquid crystal 112, and is reflected by theconductive layer 111 b. Then, the light passes through the liquidcrystal 112 and the conductive layer 113 again and reaches thepolarizing plate 130. In this case, alignment of the liquid crystal 112is controlled with a voltage that is applied between the conductivelayer 111 b and the conductive layer 113, and thus optical modulation oflight can be controlled. That is, the intensity of light emitted throughthe polarizing plate 130 can be controlled. Light other than one in aparticular wavelength region of the light is absorbed by the coloringlayer 131, and thus, emitted light is red light, for example.

The light-emitting element 60 is a bottom-emission light-emittingelement. The light-emitting element 60 has a structure in which aconductive layer 191, an EL layer 192, and a conductive layer 193 b arestacked in this order from the insulating layer 220 side. In addition, aconductive layer 193 a is provided to cover the conductive layer 193 b.The conductive layer 193 b contains a material reflecting visible light,and the conductive layers 191 and 193 a contain a material transmittingvisible light. Light is emitted from the light-emitting element 60 tothe substrate 61 side through the coloring layer 134, the insulatinglayer 220, the opening 251, the conductive layer 113, and the like.

Here, as illustrated in FIG. 7, the conductive layer 111 a transmittingvisible light is preferably provided for the opening 251. Accordingly,the liquid crystal 112 is aligned in a region overlapping with theopening 251 as well as in the other regions, in which case an alignmentdefect of the liquid crystal is prevented from being generated in theboundary portion of these regions and undesired light leakage can besuppressed.

As the polarizing plate 130 provided on an outer surface of thesubstrate 61, a linear polarizing plate or a circularly polarizing platecan be used. An example of a circularly polarizing plate is a stackincluding a linear polarizing plate and a quarter-wave retardationplate. Such a structure can reduce reflection of external light. Thecell gap, alignment, drive voltage, and the like of the liquid crystalelement used as the liquid crystal element 40 are controlled dependingon the kind of the polarizing plate so that desirable contrast isobtained.

An insulating layer 217 is provided on the insulating layer 216 coveringan end portion of the conductive layer 191. The insulating layer 217 hasa function as a spacer for preventing the insulating layer 220 and thesubstrate 51 from getting closer more than necessary. In addition, inthe case where the EL layer 192 or the conductive layer 193 a is formedusing a blocking mask (metal mask), the insulating layer 217 may have afunction of preventing the blocking mask from being in contact with asurface on which the EL layer 192 or the conductive layer 193 a isformed. Note that the insulating layer 217 is not necessarily provided.

One of a source and a drain of the transistor 205 is electricallyconnected to the conductive layer 191 of the light-emitting element 60through a conductive layer 224.

One of a source and a drain of the transistor 206 is electricallyconnected to the conductive layer 111 b through a connection portion207. The conductive layers 111 b and 111 a are in contact with andelectrically connected to each other. Here, in the connection portion207, the conductive layers provided on both surfaces of the insulatinglayer 220 are connected to each other through openings in the insulatinglayer 220.

A connection portion 204 is provided in a region where the substrates 51and 61 do not overlap with each other. The connection portion 204 has astructure similar to that of the connection portion 207. On the topsurface of the connection portion 204, a conductive layer obtained byprocessing the same conductive film as the conductive layer 111 a isexposed. Thus, the connection portion 204 and the FPC 72 can beelectrically connected to each other through the connection layer 242.

A connection portion 252 is provided in part of a region where theadhesive layer 141 is provided. In the connection portion 252, theconductive layer obtained by processing the same conductive film as theconductive layer 111 a is electrically connected to part of theconductive layer 113 with a connector 243. Accordingly, a signal or apotential input from the FPC 72 connected to the substrate 51 side canbe supplied to the conductive layer 113 formed on the substrate 61 sidethrough the connection portion 252.

As the connector 243, a conductive particle can be used, for example. Asthe conductive particle, a particle of an organic resin, silica, or thelike coated with a metal material can be used. It is preferable to usenickel or gold as the metal material because contact resistance can bedecreased. It is also preferable to use a particle coated with layers oftwo or more kinds of metal materials, such as a particle coated withnickel and further with gold. As the connector 243, a material capableof elastic deformation or plastic deformation is preferably used. Asillustrated in FIG. 7, the connector 243 which is the conductiveparticle has a shape that is vertically crushed in some cases. With thecrushed shape, the contact area between the connector 243 and aconductive layer electrically connected to the connector 243 can beincreased, thereby reducing contact resistance and suppressing thegeneration of problems such as disconnection.

The connector 243 is preferably provided so as to be covered with theadhesive layer 141. For example, the connector 243 is dispersed in theadhesive layer 141 before curing of the adhesive layer 141.

FIG. 7 illustrates an example of the circuit 64 in which the transistor201 is provided.

The structure in which the semiconductor layer 231 where a channel isformed is provided between two gates is used as an example of thetransistors 201 and 205 in FIG. 7. One gate is formed using theconductive layer 221 and the other gate is formed using a conductivelayer 223 overlapping with the semiconductor layer 231 with theinsulating layer 212 provided therebetween. Such a structure enablescontrol of the threshold voltage of a transistor. In that case, the twogate electrodes may be connected to each other and supplied with thesame signal to operate the transistor. Such a transistor can have higherfield-effect mobility and thus have higher on-state current than othertransistors. Consequently, a circuit capable of high-speed operation canbe obtained. Furthermore, the area occupied by a circuit portion can bereduced. The use of the transistor having high on-state current canreduce signal delay in wirings and can reduce display unevenness even ina display panel in which the number of wirings is increased because ofincrease in size or resolution.

Note that the transistor included in the circuit 64 and the transistorincluded in the display portion 62 may have the same structure. Aplurality of transistors included in the circuit 64 may have the samestructure or different structures. A plurality of transistors includedin the display portion 62 may have the same structure or differentstructures.

A material through which impurities such as water or hydrogen do noteasily diffuse is preferably used for at least one of the insulatinglayers 212 and 213 which cover the transistors. That is, the insulatinglayer 212 or the insulating layer 213 can function as a barrier film.Such a structure can effectively suppress diffusion of the impuritiesinto the transistors from the outside, and a highly reliable displaypanel can be provided.

The insulating layer 121 is provided on the substrate 61 side to coverthe coloring layer 131 and the light-blocking layer 132. The insulatinglayer 121 may have a function of a planarization layer. The insulatinglayer 121 enables the conductive layer 113 to have an almost flatsurface, resulting in a uniform alignment state of the liquid crystal112.

An example of the method for manufacturing the display panel 100 isdescribed. For example, the conductive layer 111 a, the conductive layer111 b, and the insulating layer 220 are formed in order over a supportsubstrate provided with a separation layer, and the transistor 205, thetransistor 206, the light-emitting element 60, and the like are formed.Then, the substrate 51 and the support substrate are bonded with theadhesive layer 142. After that, separation is performed at the interfacebetween the separation layer and each of the insulating layer 220 andthe conductive layer 111 a, whereby the support substrate and theseparation layer are removed. Separately, the coloring layer 131, thelight-blocking layer 132, the conductive layer 113, and the like areformed over the substrate 61 in advance. Then, the liquid crystal 112 isdropped onto the substrate 51 or 61 and the substrates 51 and 61 arebonded with the adhesive layer 141, whereby the display panel 100 can bemanufactured.

A material for the separation layer can be selected such that separationat the interface with the insulating layer 220 and the conductive layer111 a occurs. In particular, it is preferable that a stacked layer of alayer including a high-melting-point metal material, such as tungsten,and a layer including an oxide of the metal material be used as theseparation layer, and a stacked layer of a plurality of layers, such asa silicon nitride layer, a silicon oxynitride layer, and a siliconnitride oxide layer be used as the insulating layer 220 over theseparation layer. The use of the high-melting-point metal material forthe separation layer can increase the formation temperature of a layerformed in a later step, which reduces impurity concentration andachieves a highly reliable display panel.

As the conductive layer 111 a, an oxide or a nitride such as a metaloxide, a metal nitride, or an oxide such as an oxide semiconductor whoseresistance is reduced is preferably used. In the case of using an oxidesemiconductor, a material in which at least one of the concentrations ofhydrogen, boron, phosphorus, nitrogen, and other impurities and thenumber of oxygen vacancies is made to be higher than those in asemiconductor layer of a transistor is used for the conductive layer 111a.

Here, FIG. 7 shows a structure in which color display is performed usingthe light-emitting element 60 that emits white light and the coloringlayer 134. In FIG. 7, the EL layer 192 is formed without being dividedbetween adjacent pixels.

FIG. 8 illustrates an example in which a light-emitting element 60 athat emits light of a predetermined color is used. In FIG. 8, thecoloring layer 134 is not provided. The EL layer 192 a is formed in anisland shape, and is divided between adjacent pixels. The EL layers 192a are separately formed so as to contain at least a differentlight-emitting material between pixels of different colors. For example,the EL layers 192 a can be formed by an evaporation method using ashadow mask such as a metal mask, a deposition method using a liquidmaterial such as an inkjet method or an imprint lithography, or thelike.

Components

The above components are described below.

Substrate

A material having a flat surface can be used as the substrate includedin the display panel. The substrate on the side from which light fromthe display element is extracted is formed using a material transmittingthe light. For example, a material such as glass, quartz, ceramics,sapphire, or an organic resin can be used.

The weight and thickness of the display panel can be decreased by usinga thin substrate. A flexible display panel can be obtained by using asubstrate that is thin enough to have flexibility.

Since the substrate through which light emission is not extracted doesnot need to have a light-transmitting property, a metal substrate or thelike can be used in addition to the above-mentioned substrates. A metalmaterial, which has high thermal conductivity, is preferable because itcan easily conduct heat to the whole substrate and accordingly canprevent a local temperature rise in the display panel. To obtainflexibility and bendability, the thickness of a metal substrate ispreferably greater than or equal to 10 μm and less than or equal to 200μm, further preferably greater than or equal to 20 μm and less than orequal to 50 μm.

Although there is no particular limitation on a material of a metalsubstrate, it is favorable to use, for example, a metal such asaluminum, copper, and nickel, an aluminum alloy, or an alloy such asstainless steel.

It is preferable to use a substrate subjected to insulation treatment,e.g., a metal substrate whose surface is oxidized or provided with aninsulating film. The insulating film may be formed by, for example, acoating method such as a spin-coating method or a dipping method, anelectrodeposition method, an evaporation method, or a sputtering method.An oxide film may be formed on the substrate surface by exposure to orheating in an oxygen atmosphere or by an anodic oxidation method or thelike.

Examples of the material that has flexibility and transmits visiblelight include glass that is thin enough to have flexibility, polyesterresins such as polyethylene terephthalate (PET) and polyethylenenaphthalate (PEN), a polyacrylonitrile resin, a polyimide resin, apolymethyl methacrylate resin, a polycarbonate (PC) resin, apolyethersulfone (PES) resin, a polyamide resin, a cycloolefin resin, apolystyrene resin, a polyamide imide resin, a polyvinyl chloride resin,and a polytetrafluoroethylene (PTFE). It is particularly preferable touse a material with a low thermal expansion coefficient, for example, amaterial with a thermal expansion coefficient lower than or equal to30×10⁻⁶ /K, such as a polyamide imide resin, a polyimide resin, or PET.A substrate in which a glass fiber is impregnated with an organic resinor a substrate whose thermal expansion coefficient is reduced by mixingan inorganic filler with an organic resin can also be used. A substrateusing such a material is lightweight, and thus a display panel usingthis substrate can also be lightweight.

In the case where a fibrous body is included in the above material, ahigh-strength fiber of an organic compound or an inorganic compound isused as the fibrous body. The high-strength fiber is specifically afiber with a high tensile elastic modulus or a fiber with a high Young'smodulus. Typical examples thereof include a polyvinyl alcohol basedfiber, a polyester based fiber, a polyamide based fiber, a polyethylenebased fiber, an aramid based fiber, a polyparaphenylene benzobisoxazolefiber, a glass fiber, and a carbon fiber. As the glass fiber, a glassfiber using E glass, S glass, D glass, Q glass, or the like can be used.These fibers may be used in a state of a woven or nonwoven fabric, and astructure body in which this fibrous body is impregnated with a resinand the resin is cured may be used as the flexible substrate. Thestructure body including the fibrous body and the resin is preferablyused as the flexible substrate, in which case the reliability againstbending or breaking due to local pressure can be increased.

Alternatively, glass, metal, or the like that is thin enough to haveflexibility can be used as the substrate. Alternatively, a compositematerial where glass and a resin material are attached to each other maybe used.

A hard coat layer (e.g., a silicon nitride layer and an aluminum oxidelayer) by which a surface of a display panel is protected from damage, alayer (e.g., an aramid resin layer) that can disperse pressure, or thelike may be stacked over the flexible substrate. Furthermore, tosuppress a decrease in lifetime of the display element due to moistureand the like, an insulating film with low water permeability may bestacked over the flexible substrate. For example, an inorganicinsulating material such as silicon nitride, silicon oxynitride, siliconnitride oxide, aluminum oxide, or aluminum nitride can be used.

The substrate may be formed by stacking a plurality of layers. When aglass layer is used, a barrier property against water and oxygen can beimproved and thus a highly reliable display panel can be provided.

Transistor

The transistor includes a conductive layer serving as a gate electrode,a semiconductor layer, a conductive layer serving as a source electrode,a conductive layer serving as a drain electrode, and an insulating layerserving as a gate insulating layer. In the above, a bottom-gatetransistor is used.

Note that there is no particular limitation on the structure of thetransistor included in the display device of one embodiment of thepresent invention. For example, a planar transistor, a staggeredtransistor, or an inverted staggered transistor may be used. A top-gatetransistor or a bottom-gate transistor may be used. Gate electrodes maybe provided above and below a channel.

There is no particular limitation on the crystallinity of asemiconductor material used for the transistors, and an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle-crystal semiconductor, or a semiconductor partly includingcrystal regions) may be used. It is preferable that a semiconductorhaving crystallinity be used, in which case deterioration of thetransistor characteristics can be suppressed.

As a semiconductor material used for the transistor, an element of Group14 (e.g., silicon or germanium), a compound semiconductor, or an oxidesemiconductor can be used, for example. Typically, a semiconductorcontaining silicon, a semiconductor containing gallium arsenide, anoxide semiconductor containing indium, or the like can be used.

In particular, an oxide semiconductor having a wider band gap thansilicon is preferably used. A semiconductor material having a wider bandgap and a lower carrier density than silicon is preferably used becausethe off-state leakage current of the transistor can be reduced.

For the semiconductor layer, it is particularly preferable to use anoxide semiconductor including a plurality of crystal parts whose c-axesare aligned substantially perpendicular to a surface on which thesemiconductor layer is formed or the top surface of the semiconductorlayer and in which a grain boundary is not observed between adjacentcrystal parts.

There is no grain boundary in such an oxide semiconductor; therefore,generation of a crack in an oxide semiconductor film which is caused bystress when a display panel is bent is prevented. Therefore, such anoxide semiconductor can be preferably used for a flexible display panelwhich is used in a bent state, or the like.

Moreover, the use of such an oxide semiconductor with crystallinity forthe semiconductor layer makes it possible to provide a highly reliabletransistor with a small change in electrical characteristics.

A transistor with an oxide semiconductor whose band gap is larger thanthat of silicon can hold electric charge accumulated in a capacitor thatis series-connected to the transistor for a long time, owing to the lowoff-state current of the transistor. When such a transistor is used fora pixel, operation of a driver circuit can be stopped while a gray scaleof each pixel is maintained. As a result, a display device withextremely low power consumption can be obtained.

The semiconductor layer preferably includes, for example, a filmrepresented by an In-M-Zn-based oxide that contains at least indium,zinc, and M (a metal such as aluminum, titanium, gallium, germanium,yttrium, zirconium, lanthanum, cerium, tin, neodymium, or hafnium). Inorder to reduce variations in electrical characteristics of thetransistor including the oxide semiconductor, the oxide semiconductorpreferably contains a stabilizer in addition to indium, zinc, and M.

Examples of the stabilizer, including metals that can be used as M, aregallium, tin, hafnium, aluminum, and zirconium. As another stabilizer,lanthanoid such as lanthanum, cerium, praseodymium, neodymium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, or lutetium can be given.

As an oxide semiconductor included in the semiconductor layer, any ofthe following can be used, for example: an In—Ga—Zn-based oxide, anIn—Al—Zn-based oxide, an In—Sn—Zn-based oxide, an In—Hf—Zn-based oxide,an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-basedoxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, anIn—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide,an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-basedoxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, anIn—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-basedoxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, and an In—Hf—Al—Zn-based oxide.

Note that here, an “In—Ga—Zn-based oxide” means an oxide containing In,Ga, and Zn as its main components, and there is no limitation on theratio of In: Ga: Zn. The In—Ga—Zn-based oxide may contain another metalelement in addition to In, Ga, and Zn.

The semiconductor layer and the conductive layer may include the samemetal elements contained in the above oxides. The use of the same metalelements for the semiconductor layer and the conductive layer can reducethe manufacturing cost. For example, when metal oxide targets with thesame metal composition are used, the manufacturing cost can be reduced,and the same etching gas or the same etchant can be used in processingthe semiconductor layer and the conductive layer. Note that even whenthe semiconductor layer and the conductive layer include the same metalelements, they have different compositions in some cases. For example, ametal element in a film is released during the manufacturing process ofthe transistor and the capacitor, which might result in different metalcompositions.

The energy gap of the oxide semiconductor contained in the semiconductorlayer is preferably 2 eV or more, further preferably 2.5 eV or more, andstill further preferably 3 eV or more. With the use of an oxidesemiconductor having such a wide energy gap, the off-state current ofthe transistor can be reduced.

In the case where the oxide semiconductor contained in the semiconductorlayer contains an In—M—Zn oxide, it is preferable that the atomic ratioof metal elements of a sputtering target used for forming a film of theIn—M—Zn oxide satisfy In≥M and Zn≥M. As the atomic ratio of metalelements of such a sputtering target, In:M:Zn =1:1:1, In:M:Zn=1:1:1.2,In:M: Zn=3:1:2, In:M:Zn=4:2:4.1 and the like are preferable. Note thatthe atomic ratio of metal elements in the formed semiconductor layervaries from the above atomic ratio of metal elements of the sputteringtarget within a range of ±40% as an error.

An oxide semiconductor film with low carrier density is used as thesemiconductor layer. For example, the semiconductor layer is an oxidesemiconductor film whose carrier density is lower than or equal to1×10¹⁷/cm³, preferably lower than or equal to 1×10¹⁵/cm³, furtherpreferably lower than or equal to 1×10¹³/cm³, still further preferablylower than or equal to 1×10¹¹/cm³, even further preferably lower than1×10¹⁰/cm³, and higher than or equal to 1×10⁻⁹/cm³. Such an oxidesemiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. The oxidesemiconductor has a low impurity concentration and a low density ofdefect states and can thus be referred to as an oxide semiconductorhaving stable characteristics.

Note that, without limitation to those described above, a material withan appropriate composition may be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of a transistor. To obtainthe required semiconductor characteristics of the transistor, it ispreferable that the carrier density, the impurity concentration, thedefect density, the atomic ratio between a metal element and oxygen, theinteratomic distance, the density, and the like of the semiconductorlayer be set to appropriate values.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor contained in the semiconductorlayer, oxygen vacancies are increased in the semiconductor layer, andthe semiconductor layer becomes n-type. Thus, the concentration ofsilicon or carbon (measured by secondary ion mass spectrometry) in thesemiconductor layer is lower than or equal to 2×10¹⁸ atoms/cm³,preferably lower than or equal to 2×10¹⁷ atoms/cm³.

Alkali metal and alkaline earth metal might generate carriers whenbonded to an oxide semiconductor, in which case the off-state current ofthe transistor might be increased. Therefore, the concentration ofalkali metal or alkaline earth metal of the semiconductor layer, whichis measured by secondary ion mass spectrometry, is lower than or equalto 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³.

When nitrogen is contained in the oxide semiconductor contained in thesemiconductor layer, electrons serving as carriers are generated and thecarrier density increases, so that the semiconductor layer easilybecomes n-type. Thus, a transistor including an oxide semiconductorwhich contains nitrogen is likely to be normally on. Hence, theconcentration of nitrogen which is measured by secondary ion massspectrometry is preferably set to lower than or equal to 5×10¹⁸atoms/cm³.

The semiconductor layer may have a non-single-crystal structure, forexample. The non-single-crystal structure includes CAAC-OS (c-axisaligned crystalline oxide semiconductor, or c-axis aligneda-b-plane-anchored crystalline oxide semiconductor), a polycrystallinestructure, a microcrystalline structure, or an amorphous structure, forexample. Among the non-single-crystal structures, an amorphous structurehas the highest density of defect states, whereas CAAC-OS has the lowestdensity of defect states.

An oxide semiconductor film having an amorphous structure has disorderedatomic arrangement and no crystalline component, for example.Alternatively, an oxide film having an amorphous structure has, forexample, an absolutely amorphous structure and no crystal part.

Note that the semiconductor layer may be a mixed film including two ormore of the following: a region having an amorphous structure, a regionhaving a microcrystalline structure, a region having a polycrystallinestructure, a region of CAAC-OS, and a region having a single-crystalstructure. The mixed film has, for example, a single-layer structure ora stacked-layer structure including two or more of the above-describedregions in some cases.

Composition of CAC-OS

Described below is the composition of a cloud-aligned composite oxidesemiconductor (CAC-OS) applicable to a transistor disclosed in oneembodiment of the present invention.

The CAC-OS refers to, for example, a composition of a material in whichelements included in an oxide semiconductor are unevenly distributed.The material including unevenly distributed elements has a size ofgreater than or equal to 0.5 nm and less than or equal to 10 nm,preferably greater than or equal to 1 nm and less than or equal to 2 nm,or a similar size. Note that in the following description of an oxidesemiconductor, a state in which one or more metal elements are unevenlydistributed and regions including the metal element(s) are mixed isreferred to as a mosaic pattern or a patch-like pattern. The region hasa size of greater than or equal to 0.5 nm and less than or equal to 10nm, preferably greater than or equal to 1 nm and less than or equal to 2nm, or a similar size.

Note that an oxide semiconductor preferably contains at least indium. Inparticular, indium and zinc are preferably contained. In addition, oneor more of aluminum, gallium, yttrium, copper, vanadium, beryllium,boron, silicon, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like may be contained.

For example, of the CAC-OS, an In—Ga—Zn oxide with the CAC composition(such an In-Ga-Zn oxide may be particularly referred to as CAC-IGZO) hasa composition in which materials are separated into indium oxide(InO_(X1), where X1 is a real number greater than 0) or indium zincoxide (In_(X2)Zn_(Y2)O_(Z2), where X2, Y2, and Z2 are real numbersgreater than 0), and gallium oxide (GaO_(X3), where X3 is a real numbergreater than 0) or gallium zinc oxide (Ga_(X4)Zn_(Y4)O_(Z4), where X4,Y4, and Z4 are real numbers greater than 0), and a mosaic pattern isformed. Then, InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) forming the mosaicpattern is evenly distributed in the film. This composition is alsoreferred to as a cloud-like composition.

That is, the CAC-OS is a composite oxide semiconductor with acomposition in which a region including GaO_(X3) as a main component anda region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main componentare mixed. Note that in this specification, for example, when the atomicratio of In to an element M in a first region is greater than the atomicratio of In to an element M in a second region, the first region hashigher In concentration than the second region.

Note that a compound including In, Ga, Zn, and O is also known as IGZO.Typical examples of IGZO include a crystalline compound represented byInGaO₃(ZnO)_(m1) (m1 is a natural number) and a crystalline compoundrepresented by In_((1+x0))Ga_((1−x0))O₃(ZnO)_(m0) (−1≤x0≤1; m0 is agiven number).

The above crystalline compounds have a single crystal structure, apolycrystalline structure, or a CAAC structure. Note that the CAACstructure is a crystal structure in which a plurality of IGZOnanocrystals have c-axis alignment and are connected in the a-b planedirection without alignment.

On the other hand, the CAC-OS relates to the material composition of anoxide semiconductor. In a material composition of a CAC-OS including In,Ga, Zn, and O, nanoparticle regions including Ga as a main component areobserved in part of the CAC-OS and nanoparticle regions including In asa main component are observed in part thereof. These nanoparticleregions are randomly dispersed to form a mosaic pattern. Therefore, thecrystal structure is a secondary element for the CAC-OS.

Note that in the CAC-OS, a stacked-layer structure including two or morefilms with different atomic ratios is not included. For example, atwo-layer structure of a film including In as a main component and afilm including Ga as a main component is not included.

A boundary between the region including GaO_(X3) as a main component andthe region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent is not clearly observed in some cases.

In the case where one or more of aluminum, yttrium, copper, vanadium,beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like are contained instead of gallium in a CAC-OS,nanoparticle regions including the selected element(s) as a maincomponent(s) are observed in part of the CAC-OS and nanoparticle regionsincluding In as a main component are observed in part of the CAC-OS, andthese nanoparticle regions are randomly dispersed to form a mosaicpattern in the CAC-OS.

The CAC-OS can be formed by a sputtering method under a condition wherea substrate is not heated intentionally. In the case where the CAC-OS isformed by a sputtering method, one or more of an inert gas (typically,argon), an oxygen gas, and a nitrogen gas is used as a deposition gas.Furthermore, the flow rate of the oxygen gas to the total flow rate ofthe deposition gas in deposition is preferably as low as possible, forexample, the flow rate of the oxygen gas is higher than equal to 0% andlower than 30%, preferably higher than equal to 0% and lower than orequal to 10%.

The CAC-OS has a characteristic in that a clear peak is not observedwhen measurement is conducted using a θ/2θ scan by an out-of-planemethod with an X-ray diffraction (XRD). That is, it is found that thereare no alignment in the a-b plane direction and no alignment in thec-axis alignment in the measured areas by the XRD.

In the CAC-OS, an electron diffraction pattern that is obtained byirradiation with an electron beam with a probe diameter of 1 nm (alsoreferred to as nanobeam electron beam) has regions with high luminancein a ring pattern and a plurality of bright spots appear in thering-like pattern. Thus, it is found from the electron diffractionpattern that the crystal structure of the CAC-OS includes ananocrystalline (nc) structure that does not show alignment in the planedirection and the cross-sectional direction.

For example, energy dispersive X-ray spectroscopy (EDX) is used toobtain EDX mapping, and according to the EDX mapping, the CAC-OS of theIn—Ga—Zn oxide has a composition in which the regions including GaO_(X3)as a main component and the regions including In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component are unevenly distributed and mixed.

The CAC-OS has a structure different from that of an IGZO compound inwhich metal elements are evenly distributed, and has characteristicsdifferent from those of the IGZO compound. That is, in the CAC-OS,regions including GaO_(X3) or the like as a main component and regionsincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component areseparated to form a mosaic pattern.

The conductivity of a region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)as a main component is higher than that of a region including GaO_(X3)or the like as a main component. In other words, when carriers flowthrough regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent, the conductivity of an oxide semiconductor is generated.Accordingly, when regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) asa main component are distributed in an oxide semiconductor like a cloud,high field-effect mobility (μ) can be achieved.

In contrast, the insulating property of a region including GaO_(X3) orthe like as a main component is higher than that of a region includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component. In other words,when regions including GaO_(X3) or the like as a main component aredistributed in an oxide semiconductor, leakage current can be suppressedand favorable switching operation can be achieved.

Accordingly, when a CAC-OS is used for a semiconductor element, theinsulating property derived from GaO_(X3) or the like and theconductivity derived from In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) complementeach other, whereby high on-state current (I_(on)) and high field-effectmobility (μ) can be achieved.

A semiconductor element including a CAC-OS has high reliability. Thus,the CAC-OS is suitably used in a variety of semiconductor devicestypified by a display.

Alternatively, silicon is preferably used as a semiconductor in which achannel of a transistor is formed. Although amorphous silicon may beused as silicon, silicon having crystallinity is particularlypreferable. For example, microcrystalline silicon, polycrystallinesilicon, single crystal silicon, or the like is preferably used. Inparticular, polycrystalline silicon can be formed at a lower temperaturethan single crystal silicon and has higher field effect mobility andhigher reliability than amorphous silicon. When such a polycrystallinesemiconductor is used for a pixel, the aperture ratio of the pixel canbe improved. Even in the case where pixels are provided at extremelyhigh resolution, a gate driver circuit and a source driver circuit canbe formed over a substrate over which the pixels are formed, and thenumber of components of an electronic device can be reduced.

The bottom-gate transistor described in this embodiment is preferablebecause the number of manufacturing steps can be reduced. When amorphoussilicon, which can be formed at a lower temperature than polycrystallinesilicon, is used for the semiconductor layer, materials with low heatresistance can be used for a wiring, an electrode, or a substrate belowthe semiconductor layer, resulting in wider choice of materials. Forexample, an extremely large glass substrate can be favorably used.Meanwhile, the top-gate transistor is preferable because an impurityregion is easily formed in a self-aligned manner and variation incharacteristics can be reduced. In that case, the use of polycrystallinesilicon, single crystal silicon, or the like is particularly preferable.

Conductive Layer

As materials for a gate, a source, and a drain of a transistor, and aconductive layer such as a wiring or an electrode included in a displaydevice, any of metals such as aluminum, titanium, chromium, nickel,copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten,or an alloy containing any of these metals as its main component can beused. A single-layer structure or multi-layer structure including a filmcontaining any of these materials can be used. For example, thefollowing structures can be given: a single-layer structure of analuminum film containing silicon, a two-layer structure in which analuminum film is stacked over a titanium film, a two-layer structure inwhich an aluminum film is stacked over a tungsten film, a two-layerstructure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, a two-layer structure in which acopper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a tungsten film, a three-layerstructure in which a titanium film or a titanium nitride film, analuminum film or a copper film, and a titanium film or a titaniumnitride film are stacked in this order, and a three-layer structure inwhich a molybdenum film or a molybdenum nitride film, an aluminum filmor a copper film, and a molybdenum film or a molybdenum nitride film arestacked in this order. Note that an oxide such as indium oxide, tinoxide, or zinc oxide may be used. Copper containing manganese ispreferably used because the controllability of a shape by etching isincreased.

As a light-transmitting conductive material, a conductive oxide such asindium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zincoxide to which gallium is added, or graphene can be used. Alternatively,a metal material such as gold, silver, platinum, magnesium, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, ortitanium, or an alloy material containing any of these metal materialscan be used. Alternatively, a nitride of the metal material (e.g.,titanium nitride) or the like may be used. In the case of using themetal material or the alloy material (or the nitride thereof), thethickness is set small enough to be able to transmit light.Alternatively, a stack of any of the above materials can be used as theconductive layer. For example, a stacked film of indium tin oxide and analloy of silver and magnesium is preferably used because theconductivity can be increased. They can also be used for conductivelayers such as a variety of wirings and electrodes included in a displaydevice, and conductive layers (e.g., conductive layers serving as apixel electrode or a common electrode) included in a display element.

Insulating Layer

Examples of an insulating material that can be used for the insulatinglayers include a resin such as acrylic or epoxy resin, a resin having asiloxane bond, and an inorganic insulating material such as siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride, oraluminum oxide.

The light-emitting element is preferably provided between a pair ofinsulating films with low water permeability, in which case impuritiessuch as water can be prevented from entering the light-emitting element,preventing a decrease in the reliability of the device.

As an insulating film with low water permeability, a film containingnitrogen and silicon (e.g., a silicon nitride film or a silicon nitrideoxide film), a film containing nitrogen and aluminum (e.g., an aluminumnitride film), or the like can be used. Alternatively, a silicon oxidefilm, a silicon oxynitride film, an aluminum oxide film, or the like maybe used.

For example, the water vapor transmittance of the insulating film withlow water permeability is lower than or equal to 1×10⁻⁵ [g/(m²·day)],preferably lower than or equal to 1×10⁻⁶ [g/(m²·day)], furtherpreferably lower than or equal to 1×10⁻⁷ [g/(m²·day)], and still furtherpreferably lower than or equal to 1×10⁻⁸ [g/(m²·day)].

Liquid Crystal Element

The liquid crystal element can employ, for example, a vertical alignment(VA) mode. Examples of the vertical alignment mode include amulti-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, and an advanced super view (ASV) mode.

The liquid crystal element can employ a variety of modes; for example,other than the VA mode, a twisted nematic (TN) mode, an in-planeswitching (IPS) mode, a fringe field switching (FFS) mode, an axiallysymmetric aligned micro-cell (ASM) mode, an optically compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, oran antiferroelectric liquid crystal (AFLC) mode can be used.

The liquid crystal element controls the transmission or non-transmissionof light utilizing an optical modulation action of a liquid crystal.Note that the optical modulation action of the liquid crystal iscontrolled by an electric field applied to the liquid crystal (includinga horizontal electric field, a vertical electric field, or an obliqueelectric field). As the liquid crystal used for the liquid crystalelement, thermotropic liquid crystal, low-molecular liquid crystal,high-molecular liquid crystal, polymer dispersed liquid crystal (PDLC),ferroelectric liquid crystal, anti-ferroelectric liquid crystal, or thelike can be used. These liquid crystal materials exhibit a cholestericphase, a smectic phase, a cubic phase, a chiral nematic phase, anisotropic phase, or the like depending on conditions.

As the liquid crystal material, either a positive liquid crystal or anegative liquid crystal may be used, and an appropriate liquid crystalmaterial can be used depending on the mode or design to be used.

An alignment film can be provided to adjust the alignment of a liquidcrystal. In the case where a horizontal electric field mode is employed,a liquid crystal exhibiting a blue phase for which an alignment film isunnecessary may be used. The blue phase is a liquid crystal phase, whichis generated just before a cholesteric phase changes into an isotropicphase when the temperature of a cholesteric liquid crystal is increased.Since the blue phase appears only in a narrow temperature range, aliquid crystal composition in which several weight percent or more of achiral material is mixed is used for the liquid crystal layer in orderto improve the temperature range. The liquid crystal compositioncontaining a liquid crystal exhibiting a blue phase and a chiralmaterial has a short response time and optical isotropy, whicheliminates the need for an alignment process and reduces the viewingangle dependence. Since the alignment film does not need to be provided,rubbing treatment is not necessary; accordingly, electrostatic dischargedamage caused by the rubbing treatment can be prevented, reducingdefects and damage of a liquid crystal display device in themanufacturing process.

The liquid crystal element may be a transmissive liquid crystal element,a reflective liquid crystal element, a semi-transmissive liquid crystalelement, or the like.

In one embodiment of the present invention, in particular, thereflective liquid crystal element can be used.

In the case where a transmissive or semi-transmissive liquid crystalelement is used, two polarizing plates are provided such that a pair ofsubstrates are sandwiched therebetween. Furthermore, a backlight isprovided on the outer side of the polarizing plate. The backlight may bea direct-below backlight or an edge-light backlight. The direct-belowbacklight including a light-emitting diode (LED) is preferably usedbecause local dimming is easily performed to improve contrast. Theedge-light type backlight is preferably used because the thickness of amodule including the backlight can be reduced.

In the case where a reflective liquid crystal element is used, apolarizing plate is provided on a display surface. In addition, a lightdiffusion plate is preferably provided on the display surface to improvevisibility.

In the case where the reflective or the semi-transmissive liquid crystalelement is used, a front light may be provided outside the polarizingplate. As the front light, an edge-light front light is preferably used.A front light including a light-emitting diode (LED) is preferably usedto reduce power consumption.

Light-Emitting Element

As the light-emitting element, a self-luminous element can be used, andan element whose luminance is controlled by current or voltage isincluded in the category of the light-emitting element. For example, anLED, an organic EL element, an inorganic EL element, or the like can beused.

The light-emitting element has a top emission structure, a bottomemission structure, a dual emission structure, or the like. A conductivefilm that transmits visible light is used as the electrode through whichlight is extracted. A conductive film that reflects visible light ispreferably used as the electrode through which light is not extracted.

The EL layer includes at least a light-emitting layer. In addition tothe light-emitting layer, the EL layer may further include one or morelayers containing any of a substance with a high hole-injectionproperty, a substance with a high hole-transport property, ahole-blocking material, a substance with a high electron-transportproperty, a substance with a high electron-injection property, asubstance with a bipolar property (a substance with a high electron- andhole-transport property), and the like.

For the EL layer, either a low-molecular compound or a high-molecularcompound can be used, and an inorganic compound may also be used. Eachof the layers included in the EL layer can be formed by any of thefollowing methods: an evaporation method (including a vacuum evaporationmethod), a transfer method, a printing method, an inkjet method, acoating method, and the like.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between a cathode and an anode, holes are injected tothe EL layer from the anode side and electrons are injected to the ELlayer from the cathode side. The injected electrons and holes arerecombined in the EL layer and a light-emitting substance contained inthe EL layer emits light.

In the case where a light-emitting element emitting white light is usedas the light-emitting element, the EL layer preferably contains two ormore kinds of light-emitting substances. For example, the two or morekinds of light-emitting substances are selected so as to emit light ofcomplementary colors to obtain white light emission. Specifically, it ispreferable to contain two or more selected from light-emittingsubstances emitting light of red (R), green (G), blue (B), yellow (Y),orange (O), and the like and light-emitting substances emitting lightcontaining two or more of spectral components of R, G, and B. Thelight-emitting element preferably emits light with a spectrum having twoor more peaks in the wavelength range of a visible light region (e.g.,350 nm to 750 nm). An emission spectrum of a material emitting lighthaving a peak in a yellow wavelength range preferably includes spectralcomponents also in green and red wavelength ranges.

A light-emitting layer containing a light-emitting material emittinglight of one color and a light-emitting layer containing alight-emitting material emitting light of another color are preferablystacked in the EL layer. For example, the plurality of light-emittinglayers in the EL layer may be stacked in contact with each other or maybe stacked with a region not including any light-emitting materialtherebetween. For example, between a fluorescent layer and aphosphorescent layer, a region containing the same material as one inthe fluorescent layer or the phosphorescent layer (for example, a hostmaterial or an assist material) and no light-emitting material may beprovided. This facilitates the manufacture of the light-emitting elementand reduces the drive voltage.

The light-emitting element may be a single element including one ELlayer or a tandem element in which a plurality of EL layers are stackedwith a charge generation layer therebetween.

The conductive film that transmits visible light can be formed using,for example, indium oxide, indium tin oxide, indium zinc oxide, zincoxide, or zinc oxide to which gallium is added. Alternatively, a film ofa metal material such as gold, silver, platinum, magnesium, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, ortitanium; an alloy containing any of these metal materials; or a nitrideof any of these metal materials (e.g., titanium nitride) can be formedthin so as to have a light-transmitting property. Alternatively, astacked film of any of the above materials can be used for theconductive layers. For example, a stacked film of indium tin oxide andan alloy of silver and magnesium is preferably used, in which caseconductivity can be increased. Further alternatively, graphene or thelike may be used.

For the conductive film that reflects visible light, for example, ametal material such as aluminum, gold, platinum, silver, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or analloy containing any of these metal materials can be used. Furthermore,lanthanum, neodymium, germanium, or the like may be added to the metalmaterial or the alloy. Alternatively, an alloy containing aluminum (analuminum alloy) such as an alloy of aluminum and titanium, an alloy ofaluminum and nickel, or an alloy of aluminum and neodymium may be used.Alternatively, an alloy containing silver such as an alloy of silver andcopper, an alloy of silver and palladium, or an alloy of silver andmagnesium may be used. An alloy containing silver and copper ispreferable because of its high heat resistance. Furthermore, when ametal film or a metal oxide film is stacked in contact with an aluminumfilm or an aluminum alloy film, oxidation can be suppressed. Examples ofa material for the metal film or the metal oxide film include titaniumand titanium oxide. Alternatively, the above conductive film thattransmits visible light and a film containing a metal material may bestacked. For example, a stack of silver and indium tin oxide, a stack ofan alloy of silver and magnesium and indium tin oxide, or the like canbe used.

Each of the electrodes can be formed by an evaporation method or asputtering method. Alternatively, a discharging method such as an inkjetmethod, a printing method such as a screen printing method, or a platingmethod may be used.

Note that the aforementioned light-emitting layer and layers containinga substance with a high hole-injection property, a substance with a highhole-transport property, a substance with a high electron-transportproperty, a substance with a high electron-injection property, and asubstance with a bipolar property may include an inorganic compound suchas a quantum dot or a high molecular compound (e.g., an oligomer, adendrimer, and a polymer). For example, used for the light-emittinglayer, the quantum dot can serve as a light-emitting material.

The quantum dot may be a colloidal quantum dot, an alloyed quantum dot,a core-shell quantum dot, a core quantum dot, or the like. The quantumdot containing elements belonging to Groups 12 and 16, elementsbelonging to Groups 13 and 15, or elements belonging to Groups 14 and16, may be used. Alternatively, the quantum dot containing an elementsuch as cadmium, selenium, zinc, sulfur, phosphorus, indium, tellurium,lead, gallium, arsenic, or aluminum may be used.

Adhesive Layer

As the adhesive layer, a variety of curable adhesives such as a reactivecurable adhesive, a thermosetting adhesive, an anaerobic adhesive, and aphotocuring adhesive such as an ultraviolet curable adhesive can beused. Examples of these adhesives include an epoxy resin, an acrylicresin, a silicone resin, a phenol resin, a polyimide resin, an imideresin, a polyvinyl chloride (PVC) resin, a polyvinyl butyral (PVB)resin, and an ethylene vinyl acetate (EVA) resin. In particular, amaterial with low moisture permeability, such as an epoxy resin, ispreferred. Alternatively, a two-component-mixture-type resin may beused. Further alternatively, an adhesive sheet or the like may be used.

Furthermore, the resin may include a drying agent. For example, asubstance that adsorbs water by chemical adsorption, such as oxide of analkaline earth metal (e.g., calcium oxide or barium oxide), can be used.Alternatively, a substance that adsorbs water by physical adsorption,such as zeolite or silica gel, may be used. The drying agent ispreferably included because it can prevent impurities such as water fromentering the element, thereby improving the reliability of the displaypanel.

In addition, it is preferable to mix a filler with a high refractiveindex or light-scattering member into the resin, in which case lightextraction efficiency can be enhanced. For example, titanium oxide,barium oxide, zeolite, zirconium, or the like can be used.

Connection Layer

As the connection layers, an anisotropic conductive film (ACF), ananisotropic conductive paste (ACP), or the like can be used.

Coloring Layer

Examples of a material that can be used for the coloring layers includea metal material, a resin material, and a resin material containing apigment or dye.

Light-Blocking Layer

Examples of a material that can be used for the light-blocking layerinclude carbon black, titanium black, a metal, a metal oxide, and acomposite oxide containing a solid solution of a plurality of metaloxides. The light-blocking layer may be a film containing a resinmaterial or a thin film of an inorganic material such as a metal.Stacked films containing the material of the coloring layer can also beused for the light-blocking layer. For example, a stacked-layerstructure of a film containing a material of a coloring layer whichtransmits light of a certain color and a film containing a material of acoloring layer which transmits light of another color can be employed.It is preferable that the coloring layer and the light-blocking layer beformed using the same material because the same manufacturing apparatuscan be used and the process can be simplified.

The above is the description of each of the components.

Manufacturing Method Example

A manufacturing method example of a display panel using a flexiblesubstrate is described.

Here, layers each including a display element, a circuit, a wiring, anelectrode, an optical member such as a coloring layer or alight-blocking layer, an insulating layer, and the like, arecollectively referred to as an element layer. The element layerincludes, for example, a display element, and may additionally include awiring electrically connected to the display element or an element suchas a transistor used in a pixel or a circuit.

In addition, here, a flexible member which supports the element layer ata stage at which the display element is completed (the manufacturingprocess is finished) is referred to as a substrate. For example, asubstrate includes an extremely thin film with a thickness greater thanor equal to 10 nm and less than or equal to 300 μm and the like.

As a method for forming an element layer over a flexible substrateprovided with an insulating surface, typically, there are two methodsshown below. One of them is to directly form an element layer over thesubstrate. The other method is to form an element layer over a supportsubstrate that is different from the substrate and then to separate theelement layer from the support substrate to be transferred to thesubstrate. Although not described in detail here, in addition to theabove two methods, there is a method in which the element layer isformed over a substrate which does not have flexibility and thesubstrate is thinned by polishing or the like to have flexibility.

In the case where a material of the substrate can withstand heatingtemperature in a process for forming the element layer, it is preferablethat the element layer be formed directly over the substrate, in whichcase a manufacturing process can be simplified. At this time, theelement layer is preferably formed in a state where the substrate isfixed to the support substrate, in which case transfer thereof in anapparatus and between apparatuses can be easy.

In the case of employing the method in which the element layer is formedover the support substrate and then transferred to the substrate, first,a separation layer and an insulating layer are stacked over the supportsubstrate, and then the element layer is formed over the insulatinglayer. Next, the element layer is separated from the support substrateand then transferred to the substrate. At this time, selected is amaterial with which separation at an interface between the supportsubstrate and the separation layer, at an interface between theseparation layer and the insulating layer, or in the separation layeroccurs. With the method, it is preferable that a material having highheat resistance be used for the support substrate or the separationlayer, in which case the upper limit of the temperature applied when theelement layer is formed can be increased, and an element layer includinga higher reliable element can be formed.

For example, it is preferable that a stack of a layer containing ahigh-melting-point metal material, such as tungsten, and a layercontaining an oxide of the metal material be used as the separationlayer, and a stack of a plurality of layers, such as a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, and asilicon nitride oxide layer be used as the insulating layer over theseparation layer. Note that in this specification, oxynitride containsmore oxygen than nitrogen, and nitride oxide contains more nitrogen thanoxygen.

As the method for separating the support substrate from the elementlayer, applying mechanical force, etching the separation layer, andmaking a liquid permeate the separation interface are given as examples.Alternatively, separation may be performed by heating or cooling thesupport substrate by utilizing a difference in thermal expansioncoefficient of two layers which form the separation interface.

The separation layer is not necessarily provided in the case where theseparation can be performed at an interface between the supportsubstrate and the insulating layer.

For example, glass and an organic resin such as polyimide can be used asthe support substrate and the insulating layer, respectively. In thatcase, a separation trigger may be formed by, for example, locallyheating part of the organic resin with laser light or the like, or byphysically cutting part of or making a hole through the organic resinwith a sharp tool, so that separation may be performed at an interfacebetween the glass and the organic resin.

Alternatively, a heat generation layer may be provided between thesupport substrate and the insulating layer formed of an organic resin,and separation may be performed at an interface between the heatgeneration layer and the insulating layer by heating the heat generationlayer. As the heat generation layer, any of a variety of materials suchas a material which generates heat by feeding current, a material whichgenerates heat by absorbing light, and a material which generates heatby applying a magnetic field can be used. For example, for the heatgeneration layer, a material selected from a semiconductor, a metal, andan insulator can be used.

In the aforementioned methods, the insulating layer formed of an organicresin can be used as a substrate after the separation.

The above is the description of a manufacturing method of a flexibledisplay panel.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 3

In this embodiment, an example of a transistor that can be used as thetransistors described in the above embodiments is described withreference to drawings.

The display device of one embodiment of the present invention can befabricated by using a transistor with any of various modes, such as abottom-gate transistor or a top-gate transistor. Therefore, a materialfor a semiconductor layer or the structure of a transistor can be easilychanged in accordance with the existing production line.

Bottom-Gate Transistor

FIG. 9A1 is a cross-sectional view of a transistor 810 that is achannel-protective transistor, which is a type of bottom-gatetransistor. In FIG. 9A1, the transistor 810 is formed over a substrate771. The transistor 810 includes an electrode 746 over the substrate 771with an insulating layer 772 provided therebetween. The transistor 810includes a semiconductor layer 742 over the electrode 746 with aninsulating layer 726 provided therebetween. The electrode 746 can serveas a gate electrode. The insulating layer 726 can serve as a gateinsulating layer.

The transistor 810 includes an insulating layer 741 over a channelformation region in the semiconductor layer 742. The transistor 810includes an electrode 744 a and an electrode 744 b which are partly incontact with the semiconductor layer 742 and over the insulating layer726. The electrode 744 a can serve as one of a source electrode and adrain electrode. The electrode 744 b can serve as the other of thesource electrode and the drain electrode. Part of the electrode 744 aand part of the electrode 744 b are formed over the insulating layer741.

The insulating layer 741 can serve a channel protective layer. With theinsulating layer 741 provided over the channel formation region, thesemiconductor layer 742 can be prevented from being exposed at the timeof forming the electrodes 744 a and 744 b. Thus, the channel formationregion in the semiconductor layer 742 can be prevented from being etchedat the time of forming the electrodes 744 a and 744 b. According to oneembodiment of the present invention, a transistor with favorableelectrical characteristics can be provided.

The transistor 810 includes an insulating layer 728 over the electrode744 a, the electrode 744 b, and the insulating layer 741 and furtherincludes an insulating layer 729 over the insulating layer 728.

The insulating layer 772 can be formed using a material and a methodsimilar to those of insulating layers 722 and 705. Note that theinsulating layer 772 may be formed of a stack of insulating layers. Forexample, the semiconductor layer 742 can be formed using a material anda method similar to those of the semiconductor layer 708. Note that thesemiconductor layer 742 may be formed of a stack of semiconductorlayers. For example, the electrode 746 can be formed using a materialand a method similar to those of the electrode 706. Note that theelectrode 746 may be formed of a stack of conductive layers. Theinsulating layer 726 can be formed using a material and a method similarto those of the insulating layer 707. Note that the insulating layer 726may be formed of a stack of insulating layers. For example, theelectrodes 744 a and 744 b can be formed using a material and a methodsimilar to those of the electrode 714 or 715. Note that the electrodes744 a and 744 b may be formed of a stack of conductive layers. Forexample, the insulating layer 741 can be formed using a material and amethod similar to those of the insulating layer 726. Note that theinsulating layer 741 may be formed of a stack of insulating layers. Forexample, the insulating layer 728 can be formed using a material and amethod similar to those of the insulating layer 710. Note that theinsulating layer 728 may be formed of a stack of insulating layers. Forexample, the insulating layer 729 can be formed using a material and amethod similar to those of the insulating layer 711. Note that theinsulating layer 729 may be formed of a stack of insulating layers.

The electrode, the semiconductor layer, the insulating layer, and thelike used in the transistor disclosed in this embodiment can be formedusing a material and a method disclosed in any of the other embodiments.

In the case where an oxide semiconductor is used for the semiconductorlayer 742, a material capable of removing oxygen from part of thesemiconductor layer 742 to generate oxygen vacancies is preferably usedfor regions of the electrodes 744 a and 744 b that are in contact withat least the semiconductor layer 742. The carrier concentration in theregions of the semiconductor layer 742 where oxygen vacancies aregenerated is increased, so that the regions become n-type regions (n⁺layers). Accordingly, the regions can serve as a source region and adrain region. When an oxide semiconductor is used for the semiconductorlayer 742, examples of the material capable of removing oxygen from thesemiconductor layer 742 to generate oxygen vacancies include tungstenand titanium.

Formation of the source region and the drain region in the semiconductorlayer 742 makes it possible to reduce the contact resistance between thesemiconductor layer 742 and each of the electrodes 744 a and 744 b.Accordingly, the electric characteristics of the transistor, such as thefield-effect mobility and the threshold voltage, can be favorable.

In the case where a semiconductor such as silicon is used for thesemiconductor layer 742, a layer that serves as an n-type semiconductoror a p-type semiconductor is preferably provided between thesemiconductor layer 742 and the electrode 744 a and between thesemiconductor layer 742 and the electrode 744 b. The layer that servesas an n-type semiconductor or a p-type semiconductor can serve as thesource region or the drain region in the transistor.

The insulating layer 729 is preferably formed using a material that canprevent or reduce diffusion of impurities into the transistor from theoutside. The insulating layer 729 is not necessarily formed.

When an oxide semiconductor is used for the semiconductor layer 742,heat treatment may be performed before and/or after the insulating layer729 is formed. The heat treatment can fill oxygen vacancies in thesemiconductor layer 742 by diffusing oxygen contained in the insulatinglayer 729 or other insulating layers into the semiconductor layer 742.Alternatively, the insulating layer 729 may be formed while the heattreatment is performed, so that oxygen vacancies in the semiconductorlayer 742 can be filled.

Note that a CVD method can be generally classified into a plasmaenhanced CVD (PECVD) method using plasma, a thermal CVD (TCVD) methodusing heat, and the like. A CVD method can be further classified into ametal CVD (MCVD) method, a metal organic CVD (MOCVD) method, and thelike according to a source gas to be used.

Furthermore, an evaporation method can be generally classified into aresistance heating evaporation method, an electron beam evaporationmethod, a molecular beam epitaxy (MBE) method, a pulsed laser deposition(PLD) method, an ion beam assisted deposition (IBAD) method, an atomiclayer deposition (ALD) method, and the like.

By using the PECVD method, a high-quality film can be formed at arelatively low temperature. By using a deposition method that does notuse plasma for deposition, such as an MOCVD method or an evaporationmethod, a film with few defects can be formed because damage is noteasily caused on a surface on which the film is deposited.

A sputtering method is generally classified into a DC sputtering method,a magnetron sputtering method, an RF sputtering method, an ion beamsputtering method, an electron cyclotron resonance (ECR) sputteringmethod, a facing-target sputtering method, and the like.

In the facing-target sputtering method, plasma is confined betweentargets; thus, plasma damage to a substrate can be reduced. Furthermore,step coverage can be improved because the incident angle of a sputteredparticle to a substrate can be made smaller depending on the inclinationof the target.

A transistor 811 illustrated in FIG. 9A2 is different from thetransistor 810 in that an electrode 723 that can serve as a back gateelectrode is provided over the insulating layer 729. The electrode 723can be formed using a material and a method similar to those of theelectrode 746.

In general, the back gate electrode is formed using a conductive layerand positioned so that a channel formation region of a semiconductorlayer is positioned between the gate electrode and the back gateelectrode. Thus, the back gate electrode can function in a mannersimilar to that of the gate electrode. The potential of the back gateelectrode may be the same as that of the gate electrode or may be aground (GND) potential or a predetermined potential. By changing thepotential of the back gate electrode independently of the potential ofthe gate electrode, the threshold voltage of the transistor can bechanged.

The electrode 746 and the electrode 723 can each serve as a gateelectrode. Thus, the insulating layers 726, 728, and 729 can each serveas a gate insulating layer. The electrode 723 may also be providedbetween the insulating layers 728 and 729.

In the case where one of the electrodes 746 and 723 is referred to as a“gate electrode”, the other is referred to as a “back gate electrode”.For example, in the transistor 811, in the case where the electrode 723is referred to as a “gate electrode”, the electrode 746 is referred toas a “back gate electrode”. In the case where the electrode 723 is usedas a “gate electrode”, the transistor 811 can be regarded as a kind oftop-gate transistor. Alternatively, one of the electrodes 746 and 723may be referred to as a “first gate electrode”, and the other may bereferred to as a “second gate electrode”.

By providing the electrodes 746 and 723 with the semiconductor layer 742provided therebetween and setting the potentials of the electrodes 746and 723 to be the same, a region of the semiconductor layer 742 throughwhich carriers flow is enlarged in the film thickness direction; thus,the number of transferred carriers is increased. As a result, theon-state current and field-effect mobility of the transistor 811 areincreased.

Therefore, the transistor 811 has a high on-state current for its area.That is, the area of the transistor 811 can be small for a requiredon-state current. According to one embodiment of the present invention,the area occupied by a transistor can be reduced. Therefore, accordingto one embodiment of the present invention, a semiconductor devicehaving a high degree of integration can be provided.

The gate electrode and the back gate electrode are formed usingconductive layers and thus each have a function of preventing anelectric field generated outside the transistor from influencing thesemiconductor layer in which the channel is formed (in particular, anelectric field blocking function against static electricity and thelike). When the back gate electrode is formed larger than thesemiconductor layer such that the semiconductor layer is covered withthe back gate electrode, the electric field blocking function can beenhanced.

Since the electrodes 746 and 723 each have a function of blocking anelectric field generated outside, electric charge of charged particlesand the like generated on the insulating layer 772 side or above theelectrode 723 do not influence the channel formation region in thesemiconductor layer 742. Thus, degradation by a stress test (e.g., anegative gate bias temperature (−GBT) stress test in which negativeelectric charge is applied to a gate) can be reduced. Furthermore, achange in gate voltage (rising voltage) at which on-state current startsflowing depending on drain voltage can be reduced. Note that this effectis obtained when the electrodes 746 and 723 have the same potential ordifferent potentials.

The BT stress test is one kind of acceleration test and can evaluate, ina short time, a change by long-term use (i.e., a change over time) incharacteristics of a transistor. In particular, the amount of change inthe threshold voltage of a transistor before and after the BT stresstest is an important indicator when examining the reliability of thetransistor. As the change in threshold voltage is smaller, thetransistor has higher reliability.

By providing the electrodes 746 and 723 and setting the potentials ofthe electrodes 746 and 723 to be the same, the amount of change inthreshold voltage is reduced. Accordingly, variations in electricalcharacteristics among a plurality of transistors are also reduced.

A transistor including a back gate electrode has a smaller change inthreshold voltage before and after a positive GBT stress test, in whichpositive electric charge is applied to a gate, than a transistorincluding no back gate electrode.

When the back gate electrode is formed using a light-blocking conductivefilm, light can be prevented from entering the semiconductor layer fromthe back gate electrode side. Therefore, photodegradation of thesemiconductor layer can be prevented, and deterioration in electricalcharacteristics of the transistor, such as a shift of the thresholdvoltage, can be prevented.

According to one embodiment of the present invention, a transistor withhigh reliability can be provided. Moreover, a semiconductor device withhigh reliability can be provided.

FIG. 9B1 is a cross-sectional view of a channel-protective transistor820 that is a type of bottom-gate transistor. The transistor 820 hassubstantially the same structure as the transistor 810 but is differentfrom the transistor 810 in that the insulating layer 741 covers an endportion of the semiconductor layer 742. The semiconductor layer 742 iselectrically connected to the electrode 744 a through an opening formedby selectively removing part of the insulating layer 741 which overlapswith the semiconductor layer 742. The semiconductor layer 742 iselectrically connected to the electrode 744 b through another openingformed by selectively removing part of the insulating layer 741 whichoverlaps with the semiconductor layer 742. A region of the insulatinglayer 741 which overlaps with the channel formation region can serve asa channel protective layer.

A transistor 821 illustrated in FIG. 9B2 is different from thetransistor 820 in that the electrode 723 that can serve as a back gateelectrode is provided over the insulating layer 729.

With the insulating layer 741, the semiconductor layer 742 can beprevented from being exposed at the time of forming the electrodes 744 aand 744 b. Thus, the semiconductor layer 742 can be prevented from beingreduced in thickness at the time of forming the electrodes 744 a and 744b.

The length between the electrode 744 a and the electrode 746 and thelength between the electrode 744 b and the electrode 746 in thetransistors 820 and 821 are larger than those in the transistors 810 and811. Thus, the parasitic capacitance generated between the electrode 744a and the electrode 746 can be reduced. Moreover, the parasiticcapacitance generated between the electrode 744 b and the electrode 746can be reduced. According to one embodiment of the present invention, atransistor with favorable electrical characteristics can be provided.

A transistor 825 illustrated in FIG. 9C1 is a channel-etched transistorthat is a type of bottom-gate transistor. In the transistor 825, theelectrodes 744 a and 744 b are formed without providing the insulatinglayer 741. Thus, part of the semiconductor layer 742 that is exposed atthe time of forming the electrodes 744 a and 744 b is etched in somecases. However, since the insulating layer 741 is not provided, theproductivity of the transistor can be increased.

A transistor 826 illustrated in FIG. 9C2 is different from thetransistor 825 in that the electrode 723 which can serve as a back gateelectrode is provided over the insulating layer 729.

Top-Gate Transistor

FIG. 10A1 is a cross-sectional view of a transistor 830 that is a typeof top-gate transistor. The transistor 830 includes the semiconductorlayer 742 over the insulating layer 772, the electrodes 744 a and 744 bthat are over the semiconductor layer 742 and the insulating layer 772and in contact with part of the semiconductor layer 742, the insulatinglayer 726 over the semiconductor layer 742 and the electrodes 744 a and744 b, and the electrode 746 over the insulating layer 726.

Since the electrode 746 overlaps with neither the electrode 744 a northe electrode 744 b in the transistor 830, the parasitic capacitancegenerated between the electrodes 746 and 744 a and the parasiticcapacitance generated between the electrodes 746 and 744 b can bereduced. After the formation of the electrode 746, an impurity 755 isintroduced into the semiconductor layer 742 using the electrode 746 as amask, so that an impurity region can be formed in the semiconductorlayer 742 in a self-aligned manner (see FIG. 10A3). According to oneembodiment of the present invention, a transistor with favorableelectrical characteristics can be provided.

The introduction of the impurity 755 can be performed with an ionimplantation apparatus, an ion doping apparatus, or a plasma treatmentapparatus.

As the impurity 755, for example, at least one kind of element of Group13 elements and Group 15 elements can be used. In the case where anoxide semiconductor is used for the semiconductor layer 742, it ispossible to use at least one kind of element of a rare gas, hydrogen,and nitrogen as the impurity 755.

A transistor 831 illustrated in FIG. 10A2 is different from thetransistor 830 in that the electrode 723 and the insulating layer 727are included. The transistor 831 includes the electrode 723 formed overthe insulating layer 772 and the insulating layer 727 formed over theelectrode 723. The electrode 723 can serve as a back gate electrode.Thus, the insulating layer 727 can serve as a gate insulating layer. Theinsulating layer 727 can be formed using a material and a method similarto those of the insulating layer 726.

Like the transistor 811, the transistor 831 has a high on-state currentfor its area. That is, the area of the transistor 831 can be small for arequired on-state current. According to one embodiment of the presentinvention, the area occupied by a transistor can be reduced. Therefore,according to one embodiment of the present invention, a semiconductordevice having a high degree of integration can be provided.

A transistor 840 illustrated in FIG. 10B1 is a type of top-gatetransistor. The transistor 840 is different from the transistor 830 inthat the semiconductor layer 742 is formed after the formation of theelectrodes 744 a and 744 b. A transistor 841 illustrated in FIG. 10B2 isdifferent from the transistor 840 in that the electrode 723 and theinsulating layer 727 are included. In the transistors 840 and 841, partof the semiconductor layer 742 is formed over the electrode 744 a andanother part of the semiconductor layer 742 is formed over the electrode744 b.

Like the transistor 811, the transistor 841 has a high on-state currentfor its area. That is, the area of the transistor 841 can be small for arequired on-state current. According to one embodiment of the presentinvention, the area occupied by a transistor can be reduced. Therefore,according to one embodiment of the present invention, a semiconductordevice having a high degree of integration can be provided.

A transistor 842 illustrated in FIG. 11A1 is a type of top-gatetransistor. The transistor 842 is different from the transistor 830 or840 in that the electrodes 744 a and 744 b are formed after theformation of the insulating layer 729. The electrodes 744 a and 744 bare electrically connected to the semiconductor layer 742 throughopenings formed in the insulating layers 728 and 729.

Part of the insulating layer 726 that does not overlap with theelectrode 746 is removed, and the impurity 755 is introduced into thesemiconductor layer 742 using the electrode 746 and the insulating layer726 that is left as a mask, so that an impurity region can be formed inthe semiconductor layer 742 in a self-aligned manner (see FIG. 11A3).The transistor 842 includes a region where the insulating layer 726extends beyond an end portion of the electrode 746. The semiconductorlayer 742 in a region into which the impurity 755 is introduced throughthe insulating layer 726 has a lower impurity concentration than thesemiconductor layer 742 in a region into which the impurity 755 isintroduced without through the insulating layer 726. Thus, a lightlydoped drain (LDD) region is formed in a region which does not overlapwith the electrode 746.

A transistor 843 illustrated in FIG. 11A2 is different from thetransistor 842 in that the electrode 723 is included. The transistor 843includes the electrode 723 that is formed over the substrate 771 andoverlaps with the semiconductor layer 742 with the insulating layer 772provided therebetween. The electrode 723 can serve as a back gateelectrode.

As in a transistor 844 illustrated in FIG. 11B1 and a transistor 845illustrated in FIG. 11B2, the insulating layer 726 in a region that doesnot overlap with the electrode 746 may be completely removed.Alternatively, as in a transistor 846 illustrated in FIG. 11C1 and atransistor 847 illustrated in FIG. 11C2, the insulating layer 726 may beleft.

In the transistors 842 to 847, after the formation of the electrode 746,the impurity 755 is introduced into the semiconductor layer 742 usingthe electrode 746 as a mask, so that an impurity region can be formed inthe semiconductor layer 742 in a self-aligned manner. According to oneembodiment of the present invention, a transistor with favorableelectrical characteristics can be provided. Furthermore, according toone embodiment of the present invention, a semiconductor device having ahigh degree of integration can be provided.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 4

In this embodiment, electronic devices and lighting devices of oneembodiment of the present invention are described with reference todrawings.

Electronic devices and lighting devices can be manufactured by using thedisplay device of one embodiment of the present invention. Electronicdevices and lighting devices with low power consumption can bemanufactured by using the display device of one embodiment of thepresent invention. In addition, highly reliable electronic devices andhighly reliable lighting devices can be manufactured using the displaydevice of one embodiment of the present invention.

By applying the display device of one embodiment of the presentinvention, using a mode in which an image is displayed using reflectedlight, an electronic device capable of displaying a clear image can beprovided even in a place where external light illuminance issufficiently high (e.g., outdoors on a sunny day) or in a place whereexternal light illuminance is extremely low (e.g., during the night timeor in a dark room). Furthermore, by using light obtained by mixingreflected light and emitted light, an electronic device capable ofdisplaying an image that gives a viewer the impression of seeing anactual picture can be provided. Furthermore, by performing display usingemitted light, an electronic device capable of displaying a vivid image,a smooth moving image, or the like can be provided.

Examples of electronic devices include a television set, a desktop orlaptop personal computer, a monitor of a computer or the like, a digitalcamera, a digital video camera, a digital photo frame, a mobile phone, aportable game machine, a portable information terminal, an audioreproducing device, and a large game machine such as a pachinko machine.

The electronic device or the lighting device of one embodiment of thepresent invention can be incorporated along a curved inside/outside wallsurface of a house or a building or a curved interior/exterior surfaceof a car.

The electronic device of one embodiment of the present invention mayinclude a secondary battery. Preferably, the secondary battery iscapable of being charged by contactless power transmission.

Examples of the secondary battery include a lithium ion secondarybattery such as a lithium polymer battery (lithium ion polymer battery)using a gel electrolyte, a nickel-hydride battery, a nickel-cadmiumbattery, an organic radical battery, a lead-acid battery, an airsecondary battery, a nickel-zinc battery, and a silver-zinc battery.

The electronic device of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, an image,data, or the like can be displayed on a display portion. When theelectronic device includes an antenna and a secondary battery, theantenna may be used for contactless power transmission.

The electronic device of one embodiment of the present invention mayinclude a sensor (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, electriccurrent, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays).

The electronic device of one embodiment of the present invention canhave a variety of functions such as a function of displaying a varietyof information (e.g., a still image, a moving image, and a text image)on the display portion, a touch panel function, a function of displayinga calendar, date, time, and the like, a function of executing a varietyof software (programs), a wireless communication function, and afunction of reading out a program or data stored in a recording medium.

Furthermore, the electronic device including a plurality of displayportions can have a function of displaying image information mainly onone display portion while displaying text information mainly on anotherdisplay portion, a function of displaying a three-dimensional image bydisplaying images where parallax is considered on a plurality of displayportions, or the like. Furthermore, the electronic device including animage receiving portion can have a function of photographing a stillimage or a moving image, a function of automatically or manuallycorrecting a photographed image, a function of storing a photographedimage in a recording medium (an external recording medium or a recordingmedium incorporated in the electronic device), a function of displayinga photographed image on a display portion, or the like. Note that thefunctions of the electronic devices of embodiments of the presentinvention are not limited thereto, and the electronic devices can have avariety of functions.

FIGS. 12A to 12E illustrate examples of an electronic device including adisplay portion 7000 with a curved surface. The display surface of thedisplay portion 7000 is curved, and images can be displayed on thecurved display surface. The display portion 7000 may have flexibility.

The display portion 7000 can be formed using the display device or thelike of one embodiment of the present invention. One embodiment of thepresent invention makes it possible to provide a highly reliableelectronic device with low power consumption and a curved displayportion.

FIGS. 12A and 12B illustrate examples of mobile phones. A mobile phone7100 illustrated in FIG. 12A and a mobile phone 7110 illustrated in FIG.12B each include a housing 7101, the display portion 7000, operationbuttons 7103, an external connection port 7104, a speaker 7105, amicrophone 7106, and the like. The mobile phone 7110 illustrated in FIG.12B also includes a camera 7107.

Each mobile phone includes a touch sensor in the display portion 7000.Operations such as making a call and inputting a letter can be performedby touch on the display portion 7000 with a finger, a stylus, or thelike.

With the operation buttons 7103, power ON or OFF can be switched. Inaddition, types of images displayed on the display portion 7000 can beswitched; for example, switching from a mail creation screen to a mainmenu screen can be performed.

When a detection device such as a gyroscope or an acceleration sensor isprovided inside the mobile phone, the direction of display on the screenof the display portion 7000 can be automatically changed by determiningthe orientation of the mobile phone (whether the mobile phone is placedhorizontally or vertically). Furthermore, the direction of display onthe screen can be changed by touch on the display portion 7000,operation with the operation button 7103, sound input using themicrophone 7106, or the like.

FIGS. 12C and 12D illustrate examples of portable information terminals.A portable information terminal 7200 illustrated in FIG. 12C and aportable information terminal 7210 illustrated in FIG. 12D each includea housing 7201 and the display portion 7000. Each of the portableinformation terminals may also include an operation button, an externalconnection port, a speaker, a microphone, an antenna, a camera, abattery, or the like. The display portion 7000 is provided with a touchsensor. An operation of the portable information terminal can beperformed by touching the display portion 7000 with a finger, a stylus,or the like.

Each of the portable information terminals illustrated in thisembodiment functions as, for example, one or more of a telephone set, anotebook, and an information browsing system. Specifically, the portableinformation terminals each can be used as a smartphone. Each of theportable information terminals illustrated in this embodiment is capableof executing, for example, a variety of applications such as mobilephone calls, e-mailing, reading and editing texts, music reproduction,Internet communication, and a computer game.

The portable information terminals 7200 and 7210 can display characters,image information, and the like on its plurality of surfaces. Forexample, as illustrated in FIGS. 12C and 12D, three operation buttons7202 can be displayed on one surface, and information 7203 indicated bya rectangle can be displayed on another surface. FIG. 12C illustrates anexample in which information is displayed at the top of the portableinformation terminal. FIG. 12D illustrates an example in whichinformation is displayed on the side of the portable informationterminal. Information may be displayed on three or more surfaces of theportable information terminal.

Examples of the information include notification from a socialnetworking service (SNS), display indicating reception of an e-mail oran incoming call, the title of an e-mail or the like, the sender of ane-mail or the like, the date, the time, remaining battery, and thereception strength of an antenna. Alternatively, the operation button,an icon, or the like may be displayed instead of the information.

For example, a user of the portable information terminal 7200 can seethe display (here, the information 7203) on the portable informationterminal 7200 put in a breast pocket of his/her clothes.

Specifically, a caller's phone number, name, or the like of an incomingcall is displayed in a position that can be seen from above the portableinformation terminal 7200. Thus, the user can see the display withouttaking out the portable information terminal 7200 from the pocket anddecide whether to answer the call.

FIG. 12E illustrates an example of a television set. In a television set7300, the display portion 7000 is incorporated into a housing 7301.Here, the housing 7301 is supported by a stand 7303.

The television set 7300 illustrated in FIG. 12E can be operated with anoperation switch of the housing 7301 or a separate remote controller7311. The display portion 7000 may include a touch sensor, and can beoperated by touch on the display portion 7000 with a finger or the like.The remote controller 7311 may be provided with a display portion fordisplaying data output from the remote controller 7311. With operationkeys or a touch panel of the remote controller 7311, channels and volumecan be controlled and images displayed on the display portion 7000 canbe controlled.

Note that the television set 7300 is provided with a receiver, a modem,and the like. A general television broadcast can be received with thereceiver. When the television set is connected to a communicationnetwork with or without wires via the modem, one-way (from a transmitterto a receiver) or two-way (between a transmitter and a receiver orbetween receivers) data communication can be performed.

FIG. 12F illustrates an example of a lighting device having a curvedlight-emitting portion.

The light-emitting portion included in the lighting device illustratedin FIG. 12F can be manufactured using the display device or the like ofone embodiment of the present invention. According to one embodiment ofthe present invention, a highly reliable lighting device with low powerconsumption and a curved light-emitting portion can be provided.

A light-emitting portion 7411 included in a lighting device 7400illustrated in FIG. 12F has two convex-curved light-emitting portionssymmetrically placed. Thus, all directions can be illuminated with thelighting device 7400 as a center.

The light-emitting portion included in the lighting device 7400 may haveflexibility. The light-emitting portion may be fixed on a plasticmember, a movable frame, or the like so that a light-emitting surface ofthe light-emitting portion can be bent freely depending on the intendeduse.

The lighting device 7400 includes a stage 7401 provided with anoperation switch 7403 and the light-emitting portion 7411 supported bythe stage 7401.

Note that although the lighting device in which the light-emittingportion is supported by the stage is described as an example here, ahousing provided with a light-emitting portion can be fixed on a ceilingor suspended from a ceiling. Since the light-emitting surface can becurved, the light-emitting surface is curved to have a concave shape,whereby a particular region can be brightly illuminated, or thelight-emitting surface is curved to have a convex shape, whereby a wholeroom can be brightly illuminated.

FIGS. 13A to 13I illustrate examples of portable information terminalseach including a flexible and bendable display portion 7001.

The display portion 7001 is manufactured using the display device or thelike of one embodiment of the present invention. For example, a displaydevice or the like that can be bent with a radius of curvature ofgreater than or equal to 0.01 mm and less than or equal to 150 mm can beused. The display portion 7001 may include a touch sensor so that theportable information terminal can be operated by touch on the displayportion 7001 with a finger or the like. One embodiment of the presentinvention makes it possible to provide a highly reliable electronicdevice including a display portion having flexibility.

FIGS. 13A and 13B are perspective views illustrating an example of theportable information terminal. A portable information terminal 7500includes a housing 7501, the display portion 7001, a display portion tab7502, operation buttons 7503, and the like.

The portable information terminal 7500 includes a rolled flexibledisplay portion 7001 in the housing 7501. The display portion 7001 canbe pulled out by using the display portion tab 7502.

The portable information terminal 7500 can receive an image signal witha control portion incorporated therein and can display the receivedimage on the display portion 7001. The portable information terminal7500 incorporates a battery. A terminal portion for connecting aconnector may be included in the housing 7501 so that an image signaland power can be directly supplied from the outside with a wiring.

By pressing the operation buttons 7503, power ON/OFF, switching ofdisplayed images, and the like can be performed. Although FIGS. 13A and13B show an example in which the operation buttons 7503 are positionedon a side surface of the portable information terminal 7500, oneembodiment of the present invention is not limited thereto. Theoperation buttons 7503 may be placed on a display surface (a frontsurface) or a rear surface of the portable information terminal 7500.

FIG. 13B illustrates the portable information terminal 7500 in a statewhere the display portion 7001 is pulled out. Images can be displayed onthe display portion 7001 in this state. In addition, the portableinformation terminal 7500 may perform different displays in the statewhere part of the display portion 7001 is rolled as shown in FIG. 13Aand in the state where the display portion 7001 is pulled out as shownin FIG. 13B. For example, in the state shown in FIG. 13A, the rolledportion of the display portion 7001 is put in a non-display state,reducing the power consumption of the portable information terminal7500.

Note that a reinforcement frame may be provided for a side portion ofthe display portion 7001 so that the display portion 7001 has a flatdisplay surface when pulled out.

Note that in addition to this structure, a speaker may be provided forthe housing so that sound is output with an audio signal receivedtogether with an image signal.

FIGS. 13C to 13E illustrate an example of a foldable portableinformation terminal.

FIG. 13C illustrates a portable information terminal 7600 that isopened. FIG. 13D illustrates the portable information terminal 7600 thatis being opened or being folded. FIG. 13E illustrates the portableinformation terminal 7600 that is folded. The portable informationterminal 7600 is highly portable when folded, and is highly browsablewhen opened because of a seamless large display area.

The display portion 7001 is supported by three housings 7601 joinedtogether by hinges 7602. By folding the portable information terminal7600 at a connection portion between two housings 7601 with the hinges7602, the portable information terminal 7600 can be reversibly changedin shape from the opened state to the folded state.

FIGS. 13F and 13G illustrate an example of a foldable portableinformation terminal. FIG. 13F illustrates a portable informationterminal 7650 that is folded so that the display portion 7001 is on theinside. FIG. 13G illustrates the portable information terminal 7650 thatis folded so that the display portion 7001 is on the outside. Theportable information terminal 7650 includes the display portion 7001 anda non-display portion 7651. When the portable information terminal 7650is not used, the portable information terminal 7650 is folded so thatthe display portion 7001 is on the inside, whereby the display portion7001 can be prevented from being contaminated and damaged.

FIG. 13H illustrates an example of a flexible portable informationterminal. A portable information terminal 7700 includes a housing 7701and the display portion 7001. The portable information terminal 7700 mayfurther include buttons 7703 a and 7703 b which serve as input means,speakers 7704 a and 7704 b which serve as sound output means, anexternal connection port 7705, a microphone 7706, or the like. Aflexible battery 7709 can be included in the portable informationterminal 7700. The battery 7709 may be arranged to overlap with thedisplay portion 7001, for example.

The housing 7701, the display portion 7001, and the battery 7709 haveflexibility. Thus, it is easy to curve the portable information terminal7700 into a desired shape and to twist the portable information terminal7700. For example, the portable information terminal 7700 can be foldedso that the display portion 7001 is on the inside or on the outside. Theportable information terminal 7700 can be used in a rolled state. Sincethe housing 7701 and the display portion 7001 can be transformed freelyin this manner, the portable information terminal 7700 is less likely tobe broken even when the portable information terminal 7700 falls down orexternal stress is applied to the portable information terminal 7700.

The portable information terminal 7700 is lightweight and therefore canbe used conveniently in various situations. For example, the portableinformation terminal 7700 can be used in the state where the upperportion of the housing 7701 is suspended by a clip or the like, or inthe state where the housing 7701 is fixed to a wall by magnets or thelike.

FIG. 13I illustrates an example of a wrist-watch-type portableinformation terminal. A portable information terminal 7800 includes aband 7801, the display portion 7001, an input/output terminal 7802,operation buttons 7803, and the like. The band 7801 has a function as ahousing. A flexible battery 7805 can be included in the portableinformation terminal 7800. The battery 7805 may be arranged to overlapwith the display portion 7001, the band 7801, or the like, for example.

The band 7801, the display portion 7001, and the battery 7805 haveflexibility. Thus, the portable information terminal 7800 can be easilycurved to have a desired shape.

With the operation buttons 7803, a variety of functions such as timesetting, ON/OFF of the power, ON/OFF of wireless communication, settingand cancellation of silent mode, and setting and cancellation of powersaving mode can be performed. For example, the functions of theoperation buttons 7803 can be set freely by the operating systemincorporated in the portable information terminal 7800.

By touch on an icon 7804 displayed on the display portion 7001 with afinger or the like, application can be started.

The portable information terminal 7800 can employ near fieldcommunication conformable to a communication standard. For example,mutual communication between the portable information terminal and aheadset capable of wireless communication can be performed, and thushands-free calling is possible.

The portable information terminal 7800 may include the input/outputterminal 7802. In the case where the input/output terminal 7802 isincluded in the portable information terminal 7800, data can be directlytransmitted to and received from another information terminal via aconnector. Charging through the input/output terminal 7802 is alsopossible. Note that charging of the portable information terminaldescribed as an example in this embodiment can be performed bycontactless power transmission without using the input/output terminal.

FIG. 14A is an external view of an automobile 7900. FIG. 14B illustratesa driver's seat of the automobile 7900. The automobile 7900 includes acar body 7901, wheels 7902, a windshield 7903, lights 7904, fog lamps7905, and the like.

The display device of one embodiment of the present invention can beused in a display portion of the automobile 7900. For example, thedisplay device of one embodiment of the present invention can be used indisplay portions 7910 to 7917 illustrated in FIG. 14B.

The display portion 7910 and the display portion 7911 are provided inthe automobile windshield. The display device of one embodiment of thepresent invention can be a see-through device, through which theopposite side can be seen, by using a light-transmitting conductivematerial for its electrodes. Such a see-through display device does nothinder driver's vision during the driving of the automobile 7900.Therefore, the display device of one embodiment of the present inventioncan be provided in the windshield of the automobile 7900. Note that inthe case where a transistor or the like is provided in the displaydevice, a transistor having light-transmitting properties, such as anorganic transistor using an organic semiconductor material or atransistor using an oxide semiconductor, is preferably used.

A display portion 7912 is provided on a pillar portion. A displayportion 7913 is provided on a dashboard. For example, the displayportion 7912 can compensate for the view hindered by the pillar portionby showing an image taken by an imaging unit provided on the car body.Similarly, the display portion 7913 can compensate for the view hinderedby the dashboard and a display portion 7914 can compensate for the viewhindered by the door. That is, showing an image taken by an imaging unitprovided on the outside of the car body leads to elimination of blindareas and enhancement of safety. In addition, showing an image so as tocompensate for the area which a driver cannot see makes it possible forthe driver to confirm safety easily and comfortably.

The display portion 7917 is provided in a steering wheel. The displayportion 7915, the display portion 7916, or the display portion 7917 candisplay a variety of kinds of information such as navigation data, aspeedometer, a tachometer, a mileage, a fuel meter, a gearshiftindicator, and air-condition setting. The content, layout, or the likeof the display on the display portions can be changed freely by a useras appropriate. The information listed above can also be displayed onthe display portions 7910 to 7914.

The display portions 7910 to 7917 can also be used as lighting devices.

A display portion included in the display device of one embodiment ofthe present invention may have a flat surface. In that case, the displaydevice of one embodiment of the present invention does not necessarilyhave a curved surface and flexibility.

FIGS. 14C and 14D illustrate examples of digital signages. The digitalsignages each include a housing 8000, a display portion 8001, a speaker8003, and the like. Also, the digital signages can each include an LEDlamp, operation keys (including a power switch or an operation switch),a connection terminal, a variety of sensors, a microphone, and the like.

FIG. 14D illustrates a digital signage mounted on a cylindrical pillar.

A larger display portion 8001 can provide more information at a time. Inaddition, a larger display portion 8001 attracts more attention, so thatthe effectiveness of the advertisement is expected to be increased, forexample.

It is preferable to use a touch panel in the display portion 8001because a device with such a structure does not just display a still ormoving image on the display portion 8001, but can be operated by usersintuitively. Alternatively, in the case where the display device of oneembodiment of the present invention is used for providing informationsuch as route information or traffic information, usability can beenhanced by intuitive operation.

FIG. 14E illustrates a portable game console including a housing 8101, ahousing 8102, a display portion 8103, a display portion 8104, amicrophone 8105, a speaker 8106, an operation key 8107, a stylus 8108,and the like.

The portable game console illustrated in FIG. 14E includes two displayportions 8103 and 8104. Note that the number of display portions of anelectronic device of one embodiment of the present invention is notlimited to two and can be one or three or more as long as at least onedisplay portion includes the display device of one embodiment of thepresent invention.

FIG. 14F illustrates a laptop personal computer, which includes ahousing 8111, a display portion 8112, a keyboard 8113, a pointing device8114, and the like.

The display device of one embodiment of the present invention can beused in the display portion 8112.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

REFERENCE NUMERALS

10: display device, 11: control portion, 13: driver portion, 14: displayportion, 20: pixel unit, 21: pixel, 21B: display element, 21G: displayelement, 21R: display element, 22: pixel, 22B: display element, 22G:display element, 22R: display element, 25: light, 31: arithmeticportion, 40: liquid crystal element, 51: substrate, 60: light-emittingelement, 60 a: light-emitting element, 61: substrate, 62: displayportion, 64: circuit, 65: wiring, 72: FPC, 73: IC, 100: display panel,111 a: conductive layer, 111 b: conductive layer, 112: liquid crystal,113: conductive layer, 117: insulating layer, 121: insulating layer,130: polarizing plate, 131: coloring layer, 132: light-blocking layer,133 a: alignment film, 133 b: alignment film, 134: coloring layer, 141:adhesive layer, 142: adhesive layer, 191: conductive layer, 192: ELlayer, 192 a: EL layer, 193 a: conductive layer, 193 b: conductivelayer, 201: transistor, 204: connection portion, 205: transistor, 206:transistor, 207: connection portion, 211: insulating layer, 212:insulating layer, 213: insulating layer, 214: insulating layer, 215:insulating layer, 216: insulating layer, 217: insulating layer, 220:insulating layer, 221: conductive layer, 222: conductive layer, 223:conductive layer, 224: conductive layer, 231: semiconductor layer, 242:connection layer, 243: connector, 251: opening, 252: connection portion,311: electrode, 311 b: electrode, 340: liquid crystal element, 360:light-emitting element, 360 b: light-emitting element, 360 g:light-emitting element, 360 r: light-emitting element, 360 w:light-emitting element, 362: display portion, 400: display device, 410:pixel, 451: opening, 705: insulating layer, 706: electrode, 707:insulating layer, 708: semiconductor layer, 710: insulating layer, 711:insulating layer, 714: electrode, 715: electrode, 722: insulating layer,723: electrode, 726: insulating layer, 727: insulating layer, 728:insulating layer, 729: insulating layer, 741: insulating layer, 742:semiconductor layer, 744 a: electrode, 744 b: electrode, 746: electrode,755: impurity, 771: substrate, 772: insulating layer, 810: transistor,811: transistor, 820: transistor, 821: transistor, 825: transistor, 830:transistor, 831: transistor, 840: transistor, 841: transistor, 842:transistor, 843: transistor, 844: transistor, 845: transistor, 846:transistor, 847: transistor, 7000: display portion, 7001: displayportion, 7100: mobile phone, 7101: housing, 7103: operation button,7104: external connection port, 7105: speaker, 7106: microphone, 7107:camera, 7110: mobile phone, 7200: portable information terminal, 7201:housing, 7202: operation button, 7203: information, 7210: portableinformation terminal, 7300: television set, 7301: housing, 7303: stand,7311: remote controller, 7400: lighting device, 7401: stage, 7403:operation switch, 7411: light-emitting portion, 7500: portableinformation terminal, 7501: housing, 7502: display portion tab, 7503:operation button, 7600: portable information terminal, 7601: housing,7602: hinge, 7650: portable information terminal, 7651: non-displayportion, 7700: portable information terminal, 7701: housing, 7703 a:button, 7703 b: button, 7704 a: speaker, 7704 b: speaker, 7705: externalconnection port, 7706: microphone, 7709: battery, 7800: portableinformation terminal, 7801: band, 7802: input/output terminal, 7803:operation button, 7804: icon, 7805: battery, 7900: automobile, 7901: carbody, 7902: wheel, 7903: windshield, 7904: light, 7905: fog lamp, 7910:display portion, 7911: display portion, 7912: display portion, 7913:display portion, 7914: display portion, 7915: display portion, 7916:display portion, 7917: display portion, 8000: housing, 8001: displayportion, 8003: speaker, 8101: housing, 8102: housing, 8103: displayportion, 8104: display portion, 8105: microphone, 8106: speaker, 8107:operation key, 8108: stylus, 8111: housing, 8112: display portion, 8113:keyboard, 8114: pointing device.

This application is based on Japanese Patent Application serial no.2016-121002 filed with Japan Patent Office on Jun. 17, 2016, the entirecontents of which are hereby incorporated by reference.

1. A method for driving a display device, the display device comprising:a first display element configured to reflect visible light; and asecond display element configured to emit visible light, wherein whenthe first display element and the second display element are drivenconcurrently to display an image, a maximum value of luminance of lightemitted from the second display element is greater than or equal to 5%and less than or equal to 50% of a maximum luminance of the seconddisplay element.